WO2004112158A1

WO2004112158A1 - Piezoelectric actuator and excitation method

Info

Publication number: WO2004112158A1
Application number: PCT/IB2004/050836
Authority: WO
Inventors: Felix G. P. Peeters; Meindert L. Norg; Cornelis S. Kooijman
Original assignee: Koninklijke Philips Electronics N.V.
Priority date: 2003-06-13
Filing date: 2004-06-03
Publication date: 2004-12-23

Abstract

The invention relates to a piezoelectric actuator (11) and a method to excite a piezoelectric actuator (11), comprising at least one elongated element (2) of piezoelectric material and electrodes (13, 14, 16, 17, 18) deposited on the element (2) to displace the element (2) in the longitudinal and transverse direction by means of the electrical excitation of the electrodes (13, 14, 16, 17, 18). The electrodes (13, 14, 16, 17, 18) are excited by applying a first electrical signal (V 1) to displace the element (2) in the longitudinal direction (L) with a predefined minimum deflection and the electrodes (13, 14, 16, 17, 18) are excited by applying a second electrical signal (V2) to displace the element (2) in the transverse direction (S).

Description

Piezoelectric actuator and excitation method

The invention relates to a piezoelectric actuator and more particularly to a method for the excitation of a piezoelectric actuator, comprising at least one elongated element of piezoelectric material and electrodes deposited on the element for the longitudinal and transverse displacement thereof by the electrical excitation of the electrodes. An actuator and excitation method of the kind are disclosed in European patent 0 633 616.

This known actuator comprises a piezoelectric element with a rectangular cross section, having a (front) face on which a number of pairs of electrodes are deposited and an opposite (rear) face on which a common electrode for the pairs of electrodes is deposited. The mechanical resonance frequencies or modes in the longitudinal and transverse direction of the piezoelectric element are thus grouped into a common frequency band.

The pairs of electrodes are subsequently arranged and electrically interconnected in such a manner that the element, after the excitation of a pair of electrodes by applying an appropriate alternating voltage or alternating current, deflects in the longitudinal direction, also called longitudinal mode, and in the transverse direction, also called s-mode, because the element undergoes an S-shaped deformation in the transverse direction.

When the two resonance frequencies do not coincide exactly, the element generates an elliptical motion in the one direction of rotation or the opposite direction of rotation, depending on which pair of electrodes is excited. This motion can be used to drive an object for example. Conversely, when the two resonance frequencies or modes coincide exactly, the element is displaced along a diagonal line in the one or the other direction in relation to the longitudinal axis of the element.

This known actuator, which is used as a driving mechanism or motor to move an object in the nanometer range, is especially characterized by an intrinsic disadvantage at relatively small deflections or amplitudes, i.e. at low driving velocities of an object.

During the excitation of the actuator of the prior art, a small deflection of the element in the transverse direction will also produce a proportionally small deflection in the longitudinal direction. This is due to the fact that the intensity of the excitation of the longitudinal mode bears a constant ratio to the strength of the excitation of the s-mode. Therefore, no resulting driving force will be exerted on an object during resonance in the s- mode, since a displacement in the one direction is followed by a displacement in the other, opposite direction. In fact, the element generates a sinusoidal transverse motion whose resultant is zero, i.e. no macroscopic motion. In order to obtain an effective resulting force in the transverse direction of the element, 'a rectifying action' should take place during the coupling of the element with an object, i.e. a force exerted on an object by the element in the s-mode in one direction is followed by a force in the same direction. To this end, the element should also generate sufficient motion in the longitudinal direction. The longitudinal motion 'decouples' the coupling of the element with the object in said other direction of the s-mode and exerts a resulting driving force on the object. Without this rectifying action, the element will not exert any resulting driving force on an object in the transverse direction.

It has been shown that the deflection in the longitudinal direction is too small to produce a rectifying action below a certain threshold or limit value of the excitation signal. Although the element continues to move, no net driving force will be exerted on the object. The element will only exert a resulting driving force on the object above a specific strength of the excitation signal. From a macroscopic point of view, this effect manifests itself as a "dead zone" in the relation between the strength of the excitation signal and the driving of the object. The extent of this dead zone depends on a number of variables, including the extent of any possible mechanical prestressing of the element, the presence of contaminants between the element and a driven object, wear and tear, differences in the production process of the element, etc.

The adverse consequences of this dead zone are considerable. When the actuator is used to drive an object, the actuation is characterized by a considerable non- linearity. Since the dead zone depends on a relatively large number of variables, full compensation is excluded. Overcompensation can for instance easily lead to instabilities in the control, and consequently, an undercompensation should be maintained. Well-known methods in the field include the adequate superposing of trigger pulses on the excitation signal etc. This requires complicated and relatively expensive control algorithms and signal generation means.

It is an object of the present invention to present an improved method for the excitation of the actuator referred to in the preamble, which effectively eliminates the dead zone. The present invention achieves this object by exciting the electrodes of the actuator by applying a first electrical signal to displace the element in the longitudinal direction with a predefined minimal deflection and by exciting the electrodes by applying a second electrical signal to displace the element in the transverse direction. The present invention is concerned with the effective elimination of the dead zone by decoupling the extent of the excitation of the longitudinal mode and the extent of excitation of the s-mode. The deflection in the longitudinal mode is defined in such a manner, that the aforementioned rectifying action is produced, regardless of the amplitude of the deflections in the transverse direction, i.e. the s-mode. The second signal in the method according to the invention may be a control signal to drive an object at the desired velocity or with the desired deflection.

The method according to the invention can be used for any type of piezoelectric actuator.

In a preferred embodiment of the invention, the element is principally driven at mechanical resonant frequency. A relatively small signal amplitude can produce a relatively large deflection of the element in the longitudinal direction during resonance, which is a major advantage where the output is concerned.

The quality or Q factor of a piezoelectric element can amount to 3500 in practice, which means, as already known, that there exists a small frequency range in which the element resonates. Excitation of the element by applying the first signal with a constant frequency adapted to the resonant frequency of the element, will not suffice in practice since relatively small deviations in the material properties of the element already produce a considerable deviation of the resonant frequency.

In a further embodiment of the invention, the frequency of the first and second excitation signal is controlled depending on the mechanical resonance of the element, for example by measuring the mechanical resonant frequency with the aid of an additional electrode or transducer coupled to the element, and using the piezoelectric generating properties of the element.

In yet another embodiment of the invention, the frequency of the first and second excitation signal is driven depending on the electrical properties of the element. In particular, the electrical impedance of the element is used as a measure of the electrical properties of the element. The imaginary portion of the impedance spectrum is zero during resonance, which can be used advantageously to determine the resonance effect. In yet another embodiment of the invention, the element is excited via a pre- switched electrical matching network. Said matching network can reduce the sensitivity of the element to variations in material properties. This applies to both the energizing of the first signal for the excitation of the longitudinal mode and the energizing of the second signal for the excitation of the s-mode.

An electrical matching network for the independent optimization of the deflection in the longitudinal and transverse direction of the element, comprises, according to the invention, the application of the first and second signal to the electrodes via a first and second inductance and a common third inductance respectively. The optimal matching of the piezoelectric element not only results in a greater robustness against variations in material properties, but also produces relatively larger deflections at the same signal amplitudes.

The invention also relates to a piezoelectric actuator, comprising at least one elongated element of piezoelectric material and electrodes deposited on the element to displace the element in the longitudinal and transverse direction by the electrical excitation of the electrodes, characterized by providing first excitation means for the excitation of the electrodes by applying a first electrical signal for the displacement in the longitudinal direction of the element with a predefined minimal deflection and by providing second excitation means for the excitation of the electrodes by applying a second electrical signal to displace the element in the transverse direction. In an embodiment of the piezoelectric actuator according to the invention, the excitation means are designed to drive the element at mechanical resonant frequency, either with or without the coupled means to the element to drive the excitation means, depending on the mechanical resonance of the element, or means to drive the excitation means, depending on the electrical properties of the element such as the electrical impedance. In a preferred embodiment of the piezoelectric actuator according to the invention, an electrical matching network has been connected between the electrodes of the element and the first and second excitation means, in particular comprising a first inductance connected between the first excitation means and the electrodes, a second inductance connected between the second excitation means and the electrodes and a common third inductance connected to the first and second excitation means and the electrodes.

This not only renders the actuator more robust, i.e. a greater robustness in comparison with the excitation into resonance, but also produces a larger deflection during resonance. In a practical application of the actuator for the driving or moving of an object for example, the second excitation means are designed to drive the displacement of the element in the transverse direction with a desired deflection. Such a drive does not produce a dead zone as in the prior art, regardless of the deflection of the elongated element in the transverse direction.

The invention will now be described with reference to the following drawings, in which: Fig. Ia and Fig. Ib are schematic diagrams which respectively show the longitudinal resonance mode and the shear resonance mode of an elongated piezoelectric element of a piezoelectric actuator.

Figures 2a and 2b show a schematic view of a first embodiment of a piezoelectric actuator used with the method according to the invention. Figures 3a and 3b show a schematic view of a second embodiment of a piezoelectric actuator used with the method according to the invention.

Figure 4 shows a schematic view of an embodiment of an electrical matching network according to the invention for the actuator according to Figure 3.

Figure 5 shows a schematic view of a further possibility to drive the actuator at resonant frequency according to the invention.

Figure 6 shows a schematic view of a further possibility to drive the actuator at resonant frequency according to the invention.

Figure 7 shows a schematic and graphic view of alternatives of the relation between the s-mode and the longitudinal mode using the method according to the invention. Figure 8 shows a schematic and graphic view of the amplitude of the excitation signal applied to a pair of electrodes of the actuator according to the invention depending on the extent of the deflection in the transverse direction.

Referring to Figures Ia and Ib, the principle of operation of the piezoelectric actuator will be explained.

Figure Ia shows a schematic view of a piezoelectric actuator I₅ comprising an elongated rectangular element 2 of piezoelectric material. The piezoelectric element 2 is in general a monolith but may be built up from a number of substrates 2a, 2b, 2c, 2d, 2e of piezoelectric material which are bonded in the direction of the longitudinal axis 3 of the element. A tip 4 thereof may be designed to drive an object for example and operates during coupling with the object. The opposite tip of the element 2 may be restrained by either passive or active clamping, using a spring or similar means for example (not shown). Figure Ia shows a broken line which represents the element 2 in excitation to generate the alternating deflection in the plane of the drawing along the longitudinal axis 3. Also called longitudinal mode, as shown schematically by arrow L.

The element 2 is composed in such a manner, that it can generate a sigmoidal deflection in a transverse direction along the longitudinal axis 3 of the element 2 during an appropriate excitation, in the present case in the plane of the drawing. Also called s-mode, as shown by the arrow S in Figure Ib. The deflections are located in the nanometer range.

Figure 2a shows an actuator 5 of the type shown in Figures Ia and Ib, comprising an elongated element 2 having one side or front face (6) on which electrodes 8, 9 are connected in series in the longitudinal direction. A common electrode 10 for electrodes 8, 9 is deposited on the opposite other or rear side 7 of the element 2. See Figure 2b.

This electrode structure is particularly suited according to the invention for the excitation of the element 2 in the longitudinal direction or longitudinal mode by supplying an appropriate alternating current to the electrodes with a frequency matched to and/or locked onto the mechanical resonant frequency in the longitudinal direction of the element 2 via the schematically shown connecting wires 80, 90 and 100.

Figure 3 a shows an actuator 1 1 of the type shown in Figures Ia and Ib, comprising an elongated element 2 having one face or front face 6 on which pairs of electrodes have been connected in series in the longitudinal and transverse direction, i.e. a first pair of electrodes with crosswise arranged electrodes 13 and 14, and a second pair of electrodes with crosswise arranged electrodes 16 and 17. A common electrode 18 for the electrodes 13, 14 is deposited on the opposite other face or rear face 7 of the element 2. See Figure 3 b. The electrodes of each pair of electrodes are electrically interconnected, as shown by the connections 12 and 15 respectively in Figure 3 a.

This electrode structure is particularly suited for the excitation of the element 2 in the longitudinal and transverse direction, i.e. the longitudinal mode and s-mode, by supplying an appropriate alternating current to the electrodes with a frequency matched to and/or locked onto the mechanical resonant frequency of the element 2 via the connecting wires 120, 150 and 180. Both resonance frequencies for the deflection of the element in the longitudinal and transverse direction have been grouped into a common frequency band for this purpose.

In the known method, both the s-mode and longitudinal mode are excited by energizing a pair of electrodes by applying an alternating current with an appropriate frequency, i.e. within the common resonant frequency band. Element 2 will generate an elliptical rotation in the one or in the other opposite direction of rotation, depending on which pair of electrodes is excited. The amplitude of the ellipse changes according to the intensity of the delivered excitation current.

The geometry of the element 2 and the electrodes 13, 14, 16 and 17 deposited on said element, creates a constant ratio between the extent of excitation of the longitudinal mode and the extent of excitation of the s-mode. A small deflection in the transverse direction (s-mode) produces a small deflection in the longitudinal direction (longitudinal mode). It is therefore possible that the deflection in the longitudinal direction in the known driving method is too small to decouple the element 2 in the direction of the longitudinal axis 3 from a driven object (not shown). The result is that the element 2 does not exert any driving force on the object as a driving force in the one direction is followed by a proportional driving force in the other, opposite direction in the s-mode. In fact, the element 2 only exerts a force on the object in the longitudinal direction and does not exert any resulting force in the transverse direction. In practice, this manifests itself as a "dead zone". In other words, an excitation signal is applied but does not produce any mechanical coupling with an object such as a wheel to drive said object.

This produces a considerable non-linearity in the low velocity range. It is not possible to fully compensate this non-linearity by appropriate excitation signals as this involves too great a risk of unstable behavior. The manufacturing tolerances and variances of the element 2 are rather high, so that the risk of overcompensation and hence unstable behavior cannot be excluded. Therefore, a certain undercompensation will have to be maintained.

The invention overcomes this problem by controlling the longitudinal and transverse deflections of the element 2 independently, i.e. by decoupling both deflections. The longitudinal mode is excited by applying a first electrical excitation signal, in such a manner that, regardless of the extent of the deflection in the transverse direction or s-mode, which is excited by applying a second electrical excitation signal, a sufficient extent of deflection in the longitudinal direction is produced for the decoupling of the element 2 from a driven object, which results in a rectifying action of the s-mode on the object. This effectively eliminates the dead zone and a relatively small second electrical signal will produce a proportionally small force on the driven object. In practice, the second electrical signal may be a control signal which is sent by a control device for the control of the movement or displacement of the driven object. In a preferred embodiment of the invention, as shown in Figure 4, the first and second excitation signal or the first and second excitations means Vl and V2 respectively, are applied to the electrodes 13, 14, 16, 17 and 18 respectively of the actuator 11 via an electrical matching network 20. A highly advantageous matching network 20 according to the invention comprises first, second and third inductances 21, 22 and 23 connected in series with each of the pairs of electrodes 13, 14 and 16, 17 and connected in series with the common electrode 18. These inductances, generally coils, may be dimensioned in such a manner that the applied first and second excitation currents Vl, V2 can be optimally adapted to the resonance properties of the element 2. This produces an optimal and even reinforced deflection of the element 2 and considerably improves the robustness against tolerances, variances etc.

The values of the inductances 21, 22 and 23 can be determined by conducting a measurement with the aid of an impedance analyzer (not shown). The imaginary portion of the measured impedance for each of the pairs of electrodes should be zero at the desired resonant frequency. The invention is in no respect limited to the matching network 20 which is illustrated and described herein. For those skilled in the art, other electrical matching networks are conceivable, which for example may comprise condensers or combinations of coils and condensers (not shown). For the sake of clarity, the earth wire of the system in the figures has been indicated by short, horizontal lines.

Instead of using an electrical matching network, a feedback signal to the first and second excitation signal Vl , V2 based on the electronic properties of the element 2 can be used. Figure 5 shows means 24 in the excitation chain to the electrodes 13, 14 and 16, 17 for the determination of the electrical properties of the element 2. For example, means 24 to detennine the instantaneous impedance of the element 2, where deviations of the impedance result in an appropriate matching of the frequency of the first and second excitation signal Vl, V2 in order to excite the element 2 optimally at its mechanical resonant frequency. The feedback signal to Vl and V2 is indicated in the schematic view of Figure by arrow 25.

The fact that the imaginary portion of the electrical impedance is zero during resonance can be used to advantage. The electrical properties of the actuator are not only determined by the type of material of the ceramic element 2, but also by the structure of the electrodes and the mechanical clamping of the element, as shown schematically by the arrows 26, 27 and a mechanical presetting acting on the element 2 for instance, as indicated in the schematic view by arrow 28 and spring 29. The short, thick lines in Figure 5 illustrate the mechanical clamping. Although not explicitly shown, the tip opposite the tip 4 of the element 2 may be restrained by passive or active clamping with the aid of a spring.

Another solution comprises the generation of a feedback signal derived from the extent of mechanical resonance, for example by depositing one or more additional electrode(s) 31 on the element 2, as shown by the actuator 30 in Figure 6. The electrical current generated between said additional electrode 31 and the common electrode 18 during resonance can be used as a measure to control 33 the frequency of the first and second excitation signals Vl and V2 with the aid of appropriate control and processing means 32. Appropriate control and processing means 32 are well known in the field and do not require any further explanation for those skilled in the art.

Figure 7 shows a graphic and schematic view of a number of control algorithms to drive a piezoelectric actuator according to the invention. As this Figure is only presented for the purpose of illustration, no specific values of variables have been plotted along the vertical and horizontal axis. The deflection L of the element 2 in the longitudinal direction (the longitudinal mode) has been plotted along the vertical axis and the deflection S in the transverse direction (s-mode) has been plotted along the horizontal axis.

According to the invention, the longitudinal mode should always exceed a minimum level, indicated in Figure 6 by reference numeral 35, to produce the aforementioned rectifying action. The control algorithm (alternative 1) indicated by 36, keeps the longitudinal deflection constant, independently of the deflection in the s-mode. The reference numeral 37 indicates a control algorithm according to the invention (alternative 2), where the longitudinal and transverse amplitudes proportionally increase or decrease during larger deflections. Conversely, for relatively small deflections in the s-mode, the longitudinal amplitude according to the invention is maintained beyond the minimum level of 35, in the present case constant, as indicated by reference numeral 38.

For the purposes of illustration, the broken lines in Figure 7 show the well- known control algorithm, indicated by reference numeral 39, where the excitation in the longitudinal and s-modes is proportional across the entire control range. An appropriate control algorithm according to the invention can be chosen depending on the application. Other control algorithms are obviously possible within the scope of the invention, where the deflection in the longitudinal direction of the piezoelectric element should always exceed the minimum level of 35. Figure 8 shows a graphical and schematic view of the amplitude of the excitation signal applied to a pair of electrodes of the actuator 11, 30 according to the invention depending on the extent of the deflection in the transverse direction, i.e. the s- mode. Since this Figure is only presented for illustration purposes, no specific values have been plotted along the vertical and horizontal axis. The amplitude of the excitation signal has been plotted along the vertical axis. The deflection or S-shaped amplitude of the actuator 11 , 30 in the transverse direction (the s-mode) has been plotted along the horizontal axis. The line 40 corresponds to the excitation of the first pair of electrodes 13, 14 and line 41 corresponds to the excitation of the second pair of electrodes 16, 17. The deflection in the longitudinal direction or longitudinal mode is obtained by adding lines 40, 41. A control algorithm can be implemented with the aid of the graphs in Figures 7 and 8.

The invention is obviously not restricted to the embodiments shown and described. It should be understood that those skilled in the art may realize many embodiments without departing from the spirit and scope of the invention as defined in the appended claims.

Claims

CLAIMS:

1. A method for the excitation of a piezoelectric actuator (5, 11, 30), comprising at least one elongated element (2) of piezoelectric material and electrodes (8, 9, 10; 13, 14, 16, 17, 18) deposited on the element (2) to displace the element (2) in the longitudinal and transverse direction by means of the electrical excitation of the electrodes (8, 9, 10; 13, 14, 16, 17, 18), characterized in that the electrodes (8, 9, 10; 13, 14, 16, 17, 18) are excited by applying a first electrical signal (Vl) to displace the element (2) in the longitudinal direction (L) with a predefined minimum deflection and in that the electrodes (8, 9, 10; 13, 14, 16, 17, 18) are excited by applying a second electrical signal (V2) to displace the element (2) in the transverse direction (S).

2. A method as claimed in claim 1 , characterized in that the second signal (V2) is a drive signal to displace the element (2) in the transverse direction (S) with a desired deflection.

3. A method as claimed in claims 1 or 2, characterized in that the element (2) is driven mechanically into resonance.

4. A method as claimed in claim 3, characterized in that the frequency of the first and second excitation signal (Vl, V2) is driven depending on the mechanical resonance of the element (2).

5. A method as claimed in claims 3 or 4, characterized in that the frequency of the first and second excitation signal (Vl, V2) is controlled (25) depending on the electrical properties of the element (2).

6. A method as claimed in claim 5, characterized in that the electrical impedance of the element (2) is used as a measure of the electrical properties thereof.

7. A method as claimed in one or more of the preceding claims, characterized in that the element (2) is excited by applying the first and second excitation signal (Vl, V2) via a pre-switched matching network (20).

8. A method as claimed in claim 7, characterized in that the first and second excitation signal (Vl , V2) are applied to the electrodes (13, 14, 16, 17, 18) via a first and second inductance (21, 22) and a common third inductance (23) respectively.

9. A piezoelectric actuator (5, 11, 30), comprising at least one elongated element (2) of piezoelectric material and electrodes (8, 9, 10; 13, 14, 16, 17, 18) deposited on the element (2) to displace the element (2) in the longitudinal and transverse direction by electrically exciting the electrodes (8, 9, 10; 13, 14, 16, 17, 18), characterized in that first excitation means (Vl) are used to excite the electrodes (8, 9, 10; 13, 14, 16, 17, 18) by applying a first electrical signal to displace the element (2) in the longitudinal direction (L) with a predefined minimum deflection (35) and in that the second excitation means (V2) are used to excite the electrodes (8, 9, 10; 13, 14, 16, 17, 18) by applying a second electrical signal to displace the element (2) in the transverse direction (S).

10. A piezoelectric actuator (5, 11, 30) as claimed in claim 9, characterized by providing excitation means (Vl, V2) to drive the element (2) into mechanical resonance.

11. A piezoelectric actuator (30) as claimed in claim 10, characterized by providing means (31, 32, 33) coupled with the element (2) to control the frequency of the excitation signals applied by the first and second excitation means (Vl, V2) depending on the mechanical resonance of the element (2).

12. A piezoelectric actuator (5, 11) as claimed in claim 10 or 11, characterized by providing means (24, 25) to control the frequency of the excitation signals applied by the first and second excitations means (Vl, V2) depending on the electrical properties of the element (2).

13. A piezoelectric actuator as claimed in claim 12, characterized by providing means (24, 25) to drive the first and second excitation means (Vl, V2) depending on the electrical properties, and to drive the first and second excitation means (Vl, V2) depending on the electrical impedance of the element (2).

14. A piezoelectric actuator (5, 1 1), as claimed in one or more of the claims 9 to 13, characterized in that a switched electrical matching network (20) is connected between the electrodes (13, 14, 16, 17, 18) and the first and second excitation means (Vl, V2).

15. A piezoelectric actuator (11) as claimed in claim 14, characterized in that the matching network (20) comprises a first inductance (21) connected between the first excitation means (Vl) and the electrodes (13, 14), a second inductance (22) connected between the second excitation means (V2) and the electrodes (16, 17) and a common third inductance (23) connected between the first and second excitations means (Vl, V2) and the electrodes (13, 14, 16, 17, 18).

16. A piezoelectric actuator (5, 11, 30) as claimed in one or more of the claims 9 to 15, characterized by providing second excitation means (V2) to drive the element (2) in the transverse direction with a desired deflection (S).