GB2572334A

GB2572334A - A device for measuring piezoelectricity

Info

Publication number: GB2572334A
Application number: GB1804808.2A
Authority: GB
Inventors: Gelling Cain Markys
Original assignee: Electrosciences Ltd
Current assignee: Electrosciences Ltd
Priority date: 2018-03-26
Filing date: 2018-03-26
Publication date: 2019-10-02
Anticipated expiration: 2038-03-26
Also published as: GB2572334B; GB201804808D0

Abstract

A device 2 for measuring a piezoelectric property of a sample 3 comprises: a first electrode 6; an actuator 4 coupled to the first electrode to move the first electrode along an axis 16; a second electrode 8 positioned along the axis and spaced apart from the first electrode; a force transducer 10 coupled to the second electrode to measure the applied force; and a charge amplifier 25 to measure the charge produced by the material 3 between the two electrodes. From the measured charge and force, the piezoelectric constant can be calculated. The actuator may include a spring, washer and piezoelectric disk (see figure 2). The first electrode may have a convex, non-conducting body with a plurality of conductive bands (see figure 3). The device may have a housing with a hinge and two jaws for holding the sample under test (see figure 5). The piezoelectric property can be calculated directly without the need for a reference material of known properties to be used.

Description

A DEVICE FOR MEASURING PIEZOELECTRICITY

FIELD OF THE INVENTION

The present invention relates to the measurement of piezoelectric properties of material, for example piezoelectric coefficients.

BACKGROUND

Piezoelectric materials are useful in various technical fields, such as robotics and mobile communications devices such as smartphones. Ways of accurately measuring the piezoelectric properties of materials are desired by manufacturers, designers, and users alike.

The piezoelectric coefficient is a fundamental parameter which governs the piezoelectric activity of a material. Typically, the piezoelectric coefficient is determined using the Berlincourt Method, where the piezoelectric response of a material under test is measured against the piezoelectric response of a reference material with a known piezoelectric coefficient.

However, determination of the piezoelectric coefficient of the reference material may itself be inaccurate. Furthermore, the piezoelectric coefficient of the reference material may change with time, e.g., due to material degradation.

SUMMARY OF THE INVENTION

The present inventors have realised that the Berlincourt Method’s dependency on the known piezoelectric coefficient of the reference material tends to lead to inaccurate or unreliable results.

The present inventors have further realised that the Berlincourt Method tends to be unsuitable when determining the piezoelectric coefficient of relatively soft materials which may deform under induced stresses.

-2The present invention tends to provide an accurate and reliable way of measuring the piezoelectric coefficient of materials, including but not limited to relatively soft materials.

In a first aspect, the present invention provides a device for measuring piezoelectricity comprising: a first electrode; an actuator coupled to the first electrode, wherein the actuator is configured to actuate the first electrode along an axis; a second electrode positioned along the axis and spaced apart from the first electrode; a force transducer coupled to the second electrode, wherein the force transducer is configured to output a force signal indicative of a force acting on the second electrode; and wherein the first and second electrodes are configured to output a charge signal indicative of a charge between the first and second electrodes.

The device may further comprise a processor configured to determine a piezoelectric coefficient based on the charge signal and the force signal. At least one of the first and second electrodes may comprise a hemispherical portion. The device may further comprise a first amplifier operatively coupled to the first and second electrodes and configured to amplify the charge signal. The device may further comprise a second amplifier operatively coupled to the force transducer and configured to amplify the force signal. The device may further comprise one or more lock-in amplifiers configured to process the charge signal and/or the force signal. The device may further comprise a driver configured to generate a drive signal for actuating the actuator. The one or more lock-in amplifiers may be arranged to receive the drive signal from the driver.

In a further aspect, the present invention provides a method of measuring a piezoelectric property of a material. The method comprises: positioning the material between opposing first and second electrode such that the electrodes contact the material; actuating, by an actuator, the first electrode along an axis, thereby to apply a stress to the material; outputting a charge signal by the first and second electrodes, wherein the charge signal is indicative of a charge between the first and second electrodes; and outputting a force signal by a force transducer operatively coupled to the second electrode, wherein the force signal is indicative of a force acting on the second electrode.

-3ln a further aspect, the present invention provides an actuation device, for example for a piezoelectric measurement device. The actuation device comprises: a casing comprising a cavity, an opening to the cavity, and a flange circumscribing the opening; a piezoelectric actuator located within the cavity, the piezoelectric actuator comprising a first side and a second side opposite to the first side, wherein the first side of the piezoelectric actuator is coupled to the flange at or proximate to an edge of the first side of the piezoelectric actuator; a washer located within the cavity, the washer comprising a first side and a second side opposite to the first side, the washer comprising one or more projections extending at least to some extent perpendicularly from the first side of the washer, wherein the one or more projections are coupled to the second side of the piezoelectric actuator at or proximate to an edge of the second side of the piezoelectric actuator; and biasing means comprising a first end and a second end opposite to the first end, the biasing means being coupled between the casing and the second side of the washer, wherein the biasing means is configured to force the washer towards the piezoelectric actuator.

The one or more projections may extend from the first side of the washer at or proximate to an edge of the first side of the washer. The biasing means may comprise a wave spring. The actuation device may further comprises a mechanical gasket disposed between the piezoelectric actuator and the flange. The actuation device may further comprise an electrode coupled to the first side of the piezoelectric actuator at or proximate to a central portion of the first side of the piezoelectric actuator.

In a further aspect, the present invention provides an electrode for a device for measuring piezoelectricity. The electrode comprises an electrically nonconductive body having a convex surface and at least one electrically conductive band disposed on the convex surface. The electrically conductive band may be a continuously loop (e.g. a circular loop) of electrically conductive material.

The convex surface may be substantially hemispherical. Each of the at least one electrically conductive band may be centred about an axis of rotational symmetry of the convex surface. The electrode may comprise a plurality of electrically conductive bands which are spaced apart from each other. The

-4electrode may further comprise an electrical connection configured to connect the electrically conductive bands in parallel.

In a further aspect, the present invention provides a device comprising: a housing comprising a first portion and a second portion, the first portion and the second portion being attached together to permit relative movement of the first portion and the second portion; a first electrode coupled to the first portion; a second electrode coupled to the second portion and position opposite to the first electrode; a first cavity located within the first portion and coupled to the first electrode, the first cavity configured to receive an actuation module for actuating the first electrode; a second cavity located within the second portion and coupled to the second electrode, the second cavity configured to receive a force transducer.

The device may further comprise an actuation module removably housed in the first cavity and configured to actuate the first electrode. The actuation module may be the actuation device according to any preceding aspect. The device may further comprise a force transducer removably housed in the second cavity and configured to measure a force acting on the second electrode. The device may further comprise a third cavity configured to receive a signal processing module. The device may further comprise a first electrical connection coupled between the first cavity and the third cavity. The device may further comprise a second electrical connection coupled between the second cavity and the third cavity. The device may further comprise a signal processing module removably housed in the third cavity and configured to process electrical signals received via the first and second electrical connections. The first portion and the second portion may be hingedly attached. The device may further comprise biasing means configured to bias the first and second electrodes towards each other. At least one of the first and second electrodes may be an electrode in accordance with any preceding aspect.

In a further aspect, the present invention provides a device according to any preceding aspect comprising an actuation device according to any preceding aspect.

-5ln a further aspect, the present invention provides a device according to any preceding aspect comprising an electrode in accordance with any preceding aspect.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 is a schematic illustration (not to scale) showing a side view of a measurement device for measuring a piezoelectric coefficient of a material;

Figure 2 is a schematic illustration (not to scale) showing a side view crosssection showing further details of an actuation module of the measurement device;

Figure 3 is a schematic illustration (not to scale) showing a side view of an electrode of the measurement device;

Figure 4 is a process flowchart showing certain steps of a method of measuring a piezoelectric coefficient of a material; and

Figure 5 is a schematic illustration (not to scale) showing a side view of a modular piezoelectric measurement device.

DETAILED DESCRIPTION

It will be appreciated that relative terms such as horizontal and vertical, top and bottom, upper and lower, above and below, front and back, and so on, are used below merely for ease of reference to the Figures, and these terms are not limiting as such, and any two differing directions or positions and so on may be implemented rather than truly horizontal and vertical, upper and lower, top and bottom, and so on.

Figure 1 is a schematic illustration (not to scale) showing a side view of a measurement device 2 for measuring a piezoelectric coefficient of a piezoelectric material, e.g. a piezoelectric polymer. This material is hereinafter referred to as a Device Under Test (DUT) and is indicated in the Figures by the reference numeral

3. In this embodiment, the DUT is a so-called “unelectroded” sample, i.e. a

-6material which has not had electrical contacts or materials disposed on its surface.

In this embodiment, the measurement device 2 comprises an actuation module 4, a first electrode 6, a second electrode 8, a force transducer 10, a base structure 12, a signal processing module 14, and a plurality of locator rods 15.

The actuation module 4 will be described in more detail later below with reference to Figure 2.

A lower surface of the actuation module 4 is attached to an upper surface of the first electrode 6. The actuation module 4 is configured to actuate the first electrode 6 up and down along a vertical actuation axis 16.

The first electrode 6 is a banded electrode which is described in more detail later below with reference to Figure 3.

In this embodiment, the first electrode 6 comprises a first elongate connection member 18 and a first end portion 19. The first elongate connection member 18 and the first end portion 19 may be integrally formed.

The first end portion 19 is substantially hemispherical in shape, with a circular planar surface opposite to a curved surface. In this embodiment, the radius of curvature of the first end portion 19 is 5mm.

The first elongate member 18 has a first end and a second end opposite to the first end. The first end of the first elongate connection member 18 is attached to the lower surface of the actuation module 4. The second end of the first elongate connection member 18 is attached to the circular planar surface of the first end portion 19.

The first electrode 6 has a longitudinal axis. The first electrode 6 has rotational symmetry about its longitudinal axis.

The first electrode 6 extends substantially vertically downwards from the lower surface of the actuation module 4.

In this embodiment, the second electrode 8 is substantially identical in shape to the first electrode 6. In this embodiment, unlike the first electrode 6 (which is described in more detail later below with reference to Figure 3), the

-7 second electrode is formed entirely of an electrically conductive material, such as a metal (e.g. copper).

The second electrode 8 comprises a second elongate connection member 20 and a second end portion 21. The second elongate connection member 20 and the second end portion 21 may be integrally formed.

The second end portion 21 is substantially hemispherical in shape, with a circular planar surface opposite to a curved surface. In this embodiment, the radius of curvature of the second end portion 21 is 5mm.

The second elongate member 20 has a first end and a second end opposite to the first end. The first end of the second elongate connection member 20 is attached to the upper surface of the force transducer 10. The second end of the second elongate connection member 20 is attached to a planar lower surface of the second end portion 21.

The second electrode 8 extends substantially vertically upwards from the upper surface of the force transducer 10.

The second electrode 8 has a longitudinal axis. The second electrode 8 has rotational symmetry about its longitudinal axis.

In this embodiment, the first and second electrodes 6, 8 are arranged opposite to, or facing, each other such that the longitudinal axes of the first and second electrodes 6, 8 are aligned. Also, the actuation axis 16 coincides with the longitudinal axes of the first and second electrodes 6, 8.

In this embodiment, the first electrode 6 and the second electrode 8 are configured to be moved relative to one another. Thus, the first electrode 6 and second electrode 8 may be moved apart from each other, thereby to allow the DUT 3 to be positioned therebetween, as shown in Figure 1.

In this embodiment, the locator rods 15 provide means of locating the actuation module 4 with respect to the base structure 12. The actuation module 4 and the base structure 12 are movable with respect to each other. Thus, the locator rods 15 provide a means of locating the first electrode 6 with respect to the second electrode 8. The locator rods 15 pass through both the actuation

-8module 4 and the base structure 12 at least to some extent. The locator rods 15 may be cylindrical in shape.

In this embodiment, the force transducer 10 is a conventional force transducer, which may for example be a microelectromechanical systems (MEMs) device. The upper surface of the force transducer 10 is attached to the second electrode 8. The lower surface of the force transducer 10 is attached to the upper surface of the base structure 12. The force transducer 10 is configured to measure the force applied to it by the second electrode 8 and output a force signal indicative of this force.

In this embodiment, the signal processing module 14 comprises a driver 22, a lock-in amplifier module 23, a differential force amplifier 24, a differential charge amplifier 25, a processor 26, and a display 28.

The driver 22 is electrically connected to the actuation module 4 and the lock-in amplifier module 23. The driver 22 is configured to generate a drive signal for driving the actuation module 4, and to transmit the drive signal to the actuation module 4. The driver 22 is further configured to transmit the drive signal to the lock-in amplifier module 23. In this embodiment, the drive signal is a sinusoidal signal which may have a frequency with an order of magnitude on the scale of Hz to kHz. The actuation module 4 is configured to actuate the first electrode 6 in accordance with the drive signal received from the driver 22.

The differential force amplifier 24 is electrically connected to the force transducer 10 such that the differential force amplifier 24 may receive the force signal from the force transducer 10. The differential force amplifier 24 is configured to amplify the force signal outputted by the force transducer 10, thereby to produce an amplified force signal. The differential force amplifier 24 is further electrically connected (e.g. by wires) to the lock-in amplifier module 23. The differential force amplifier 24 is further configured to transmit the amplified force signal to the lock-in amplifier module 23.

The differential charge amplifier 25 is electrically connected to the first and second electrodes 6, 8 by respective electrical connections. The differential charge amplifier 25 is further electrically connected to the processor 26. The

-9differential charge amplifier 25 is configured to convert the piezoelectric charge generated between the first and second electrodes 6, 8, i.e. the electrical charge across the DUT 3, into a voltage. The differential charge amplifier 25 is further configured to amplify the voltage to output a charge signal indicative of the piezoelectric charge generated between the first and second electrodes 6, 8, and to transmit the charge signal to the lock-in amplifier module 23 and the processor 26.

In this embodiment, the lock-amplifier module 23 comprises two lock-in amplifiers, namely a first lock-in amplifier and a second lock-amplifier.

The lock-in amplifier module 23 is configured to receive the charge signal, the amplified force signal, and the drive signal from the differential charge amplifier 25, the differential force amplifier 24, and driver 22 respectively. The first lock-in amplifier of the lock-amplifier module 23 is configured to determine a charge measurement using the received charge signal. The second lock-in amplifier the lock-amplifier module 23 is configured to determine a force measurement using the received amplified force signal. The first and second lockin amplifiers may additionally use the received drive signal.

The lock-in amplifier module 23 is electrically connected to the processor 26. The lock-in amplifier module 23 is further configured to transmit the determined charge and force measurements to the processor 26.

The processor 26 is configured to receive the charge and force measurements from the lock-in amplifier module 23. The processor 26 is further configured to determine a piezoelectric coefficient of the DUT 3 based on the received charge and force measurements. The processor 26 is electrically connected to the display 24 such that the determined piezoelectric coefficient may be sent from the processor 26 to the display 24.

The display 24 is configured to display the determined piezoelectric coefficient received from the processor 26, e.g. for viewing by a user of the measurement device 2.

-10The processor 26 may be configured to relay the calculated piezoelectric coefficient to a remote device using a wireless communication link such as WiFi or Bluetooth.

Figure 2 is a schematic illustration (not to scale) of a side view crosssection showing further details of the actuation module 4.

In this embodiment, the actuation module 4 comprises a casing 30, a wave spring 32, a washer 34, a piezoelectric disc actuator 36, and an O-ring 38.

In this embodiment, the casing 30 is substantially cylindrical in shape. The casing 30 defines a cavity in which the wave spring 32, the washer 34, the piezoelectric disc actuator 36, and the O-ring 38 are housed. The cavity is substantially cylindrical in shape. An opening 40 to the cavity is located at the lower surface of the casing 30. The casing 30 further comprises an inwardly directed flange 41 surrounding the opening 40 and partially defining the cavity.

In this embodiment, the wave spring 32 is a conventional wave spring. The wave spring 32 is substantially cylindrical. The wave spring 32 has a diameter which is less than the diameter of the cavity. The wave spring 32 comprises a first spring end and a second spring end opposite to the first spring end. The first spring end is in contact with an upper wall of the casing 30, the upper wall being located opposite to the opening 40. The second spring end is in contact with the upper surface of the washer 34.

In this embodiment, the washer 34 is substantially disc shaped. A diameter of the washer 34 is substantially the same as or smaller than the diameter of the cavity. The washer 34 further comprises an annular projection positioned at or proximate to the circumferential edge of the lower surface of the washer 34. The annular projection projects substantially perpendicularly from the planar lower surface of the washer 34. The washer 34 is positioned within the cavity such that the annular projection of the washer 34 is in contact with the upper surface of piezoelectric disc actuator 36. The upper surface of the washer 34 is in contact with the wave spring 32. In this embodiment, the washer 34 is in contact with the upper surface of the piezoelectric disc actuator 36 only at the annular projection. Thus, central portions of the washer 34 and the piezoelectric disc actuator 36 are

-11 spaced apart from each other, so as to not inhibit movement of the central portion of the piezoelectric disc actuator 36 relative to the washer 34.

In this embodiment, the piezoelectric disc actuator 36 is substantially disc shaped. The piezoelectric disc actuator has a diameter ranging from 20mm to 60mm. The piezoelectric disc actuator 36 has a diameter that is substantially the same as or smaller than the radius of the cavity. The piezoelectric disc actuator 36 comprises a piezoelectric material. The piezoelectric disc actuator 36 is in contact with the annular projection of the washer 34 at or proximate to the circumferential edge of its upper surface. A lower surface of the piezoelectric disc actuator 36 in contact with the O-ring 38 at or proximate to the circumferential edge of the lower surface of the piezoelectric disc actuator 36. A central portion of the lower surface of the piezoelectric disc actuator 36 is attached to the first electrode 6.

The O-ring 38 is positioned within the cavity. A lower portion of the O-ring 38 is in contact with an upper surface of the flange 41. An upper portion of the O-ring 38 is in contact with the circumferential edge of the lower surface of piezoelectric disc actuator 36.

The wave spring 32 is configured to exert a load against the washer 34, thereby forcing the annular projection of the washer 34 against the circumferential edge of the upper surface of the piezoelectric disc actuator 36. The circumferential edge of the piezoelectric disc actuator 36 is thereby forced against the O-ring 38 and the flange 41 such that the circumferential edge of the piezoelectric disc actuator 36 is securely held between the washer 34 and the Oring/flange. Thus, the circumferential edge of the piezoelectric disc actuator 36 is retained and remains substantially stationary relative to the casing 30, whereas the central portion of the piezoelectric disc actuator 36 is allowed to move so as to enable actuation of the first electrode 6 along the actuation axis 16.

In operation, the drive signal received by the actuation module from the driver 22 drives the piezoelectric disc actuator 36 and causes the central portion of the piezoelectric disc actuator 36 to move up and down, thereby moving the first electrode 6 up and down along the actuation axis 16.

-12Figure 3 is a schematic illustration (not to scale) of a side view of the first electrode 6.

The first electrode 6 is a banded electrode comprising the first elongate connection member 18, the first end portion 19, a plurality of electrically conductive bands 42, and an electrical connection 44.

The end portion 40 is made of an electrical insulator, for example, glass or sapphire.

The electrically conductive bands 42 are disposed on the curved surface of the first end portion 19. The electrically conductive bands 42 are circular bands centred about the longitudinal axis of the first electrode 6. The electrically conductive bands 42 are incrementally spaced apart from each other along the longitudinal axis of the first electrode 6. Thus, when the first electrode 6 is viewed from below in a direction along the longitudinal axis of the first electrode 6, the electrically conductive bands 42 appear to be arranged as concentric circles.

The electrically conductive bands 42 are formed from one or more electrically conductive materials, for example a metal or alloy. For example, the electrically conductive bands 42 may comprise aluminium, platinum, gold, a combination of titanium and platinum, or a combination of chromium and aluminium.

The electrical connection 44 electrically connects together all of the electrically conductive bands 42 in parallel. The electrical connection 44 further electrically connects the electrically conductive bands 42 of the first electrode 6 to the differential charge amplifier 25.

As mentioned earlier above, in this embodiment, the second electrode 8 is substantially identical in shape to the first electrode 6. However, in this embodiment the second electrode is formed entirely of an electrically conductive material, such as a metal (e.g. copper).

Figure 4 is a process flowchart showing certain steps of an embodiment of a method of measuring a piezoelectric coefficient of a material. In this embodiment, the piezoelectric coefficient of the DUT 3 is determined.

-13At step s2, a human user moves the first and second electrodes 6, 8 apart so as to allow the DUT 3 to be positioned between the first and second electrodes 6, 8.

At step s4, the human user positions the DUT 3 between the first and second end portions 19, 21 of the first and second electrodes 6, 8.

At step s6, the driver 22 sends a direct current (DC) signal to the actuation module 4. This causes the central portion of the piezoelectric disc actuator 36 to move downwards. Thus, the first electrode 6 is moved downwards into contact with the DUT 3, and is retained against the upper surface of the DUT 3. Thus, the DUT 3 is gripped or clamped between the first and second electrodes 6, 8. In other words, a DC static load is applied such that the first end portion 19 is moved into contact with the upper surface of the DUT 3 and such that the second end portion 21 is moved into contact with the lower surface of the DUT 3.

The driver 22 may also transmit the DC signal to the lock-in amplifier module 23.

At step s8, while the first electrode 6 is retained against the DUT 3 (by the DC static signal), the driver 22 sends an alternating current (AC) drive signal to the actuation module 4. The piezoelectric disc actuator 36 is actuated in accordance with this AC drive signal. In particular, the central portion of the piezoelectric disc actuator 36 oscillates up and down. The peak-to-peak amplitude of this oscillation of the central portion of the piezoelectric disc actuator 36 is relatively small, for example compared to the displacement of the piezoelectric disc actuator 36 resulting from the DC signal. For example, for a 30mm diameter piezoelectric actuator 36, the peak-to-peak amplitude of the oscillation caused by the applied AC drive signal may be about 100 micrometres. In this embodiment, during this oscillation, the first electrode 6 remains in contact with the DUT 3. In this embodiment, the peak-to-peak amplitude of the oscillation caused by the applied AC drive signal is less than the distance between adjacent electrically conductive bands 42 in the direction along the length of the first electrode 6.

-14The driver 22 also transmits the AC drive signal to the lock-in amplifier module 23.

At step s10, as a result of its actuation, the first electrode 6 applies a varying mechanical stress to the DUT 3 in accordance with the AC drive signal. In response to the applied varying mechanical stress, the DUT 3 generates electrical charge (since the DUT 3 is a piezoelectric material). Thus, an electrical charge is generated by the DUT 3 between the first and second electrodes 6, 8, and thereby between the electrical connections that connect the differential charge amplifier 25 to the first and second electrodes 6, 8. This charge varies in accordance with the drive signal. The differential charge amplifier 25 generates a charge signal indicative of the piezoelectric charge between the first and second electrodes 6, 8, and transmits the charge signal to the processor 26.

In this embodiment, the DUT 3 is a relatively soft or compliant material. Thus, contact of the electrodes 6, 8 with the DUT 3 may deform the DUT 3 at least to some extent. Such deformation of the DUT 3 may affect the piezoelectric properties of the DUT 3. For example, if the DUT 3 is plastically deformed by the electrodes 6, 8 or a too high force is applied to the DUT 3, the DUT 3 may become depoled through stress induced depoling for example. Thus, it tends to be desirable to determine how the DUT 3 is deformed by contact with the electrodes 6, 8 to ensure depoling does not occur. In this embodiment, this is determined by determining the extent to which the first electrode 6 extends into, or indents, the DUT 3.

At step s12, the driver 22 gradually (e.g. continuously) increases the DC signal to the actuation module 4. This causes the central portion of the piezoelectric disc actuator 36 to gradually be moved further downwards (i.e. further translated downwards). Thus, the first electrode 6 is gradually moved further downwards, and its indentation depth into the relatively compliant DUT 3 is gradually increased.

At step s14, as the first electrode 6 is gradually moved further downwards and further into the DUT 3, the electrically conductive bands 42 of the first electrode will, in turn, contact with the DUT 3. Contact of each of the electrically

-15conductive bands 42 with the DUT 3 causes a respective discontinuity or artefact within the charge signal generated by the differential charge amplifier 25 and received by the processor 26.

At step s16, based on the received charge signal, the processor 26 determines that a predetermined one of the plurality of electrically conductive bands 42 has been moved into contact with the DUT 3. The predetermined one of the plurality of electrically conductive bands 42 may be, for example, the second band, the third band, or the fourth band from the end of the first electrode

6. The processor 26 may determine that the predetermined one of the plurality of electrically conductive bands 42 has been moved into contact with the DUT 3 by, for example, counting the discontinuities that occur in the received charge signal. For example, the processor 26 may determine that the second band is in contact with the DUT 3 responsive to the occurrence of a second discontinuity or artefact in the received charge signal.

In this embodiment, the predetermined one of the plurality of electrically conductive bands 42 being moved into contact with the DUT 3 corresponds to the first electrode 6 extending or indenting into the DUT 3 by a desired distance. This desired distance and/or the predetermined one of the plurality of electrically conductive bands 42 may be dependent upon the DUT 3 and may be determined, for example, by experimentation. In this embodiment, the first electrode 6 extending or indenting into the DUT 3 by the desired distance is not sufficient to cause plastic deformation of the DUT 3, and is not sufficient to cause unwanted behaviour of the DUT 3, e.g. depoling of the DUT 3.

At step s18, responsive to the processor 26 determining that the predetermined one of the plurality of electrically conductive bands 42 has been moved into contact with the DUT 3, the driver 22 maintains the current DC signal to the actuation module 4. The gradual downwards movement of the first electrode 6 is stopped and the indentation depth of the first electrode 6 into the DUT 3 is maintained at the current/desired distance. Thus, the current indentation depth of the first electrode 6 into the DUT 3 is maintained, and oscillation of the first electrode 6 (caused by the applied AC drive signal) occurs about that current indentation depth.

-16At step s20, the oscillation of the first electrode 6 about the desired indentation depth causes the first electrode 6 to apply a varying mechanical stress to the DUT 3 in accordance with the AC drive signal. In response to this applied varying mechanical stress, the DUT 3 generates electrical charge which varies in accordance with the AC drive signal. The differential charge amplifier 25 generates a charge signal indicative of the piezoelectric charge between the first and second electrodes 6, 8, and transmits the charge signal to the lock-in amplifier module 23.

At step s22, the force transducer 10 measures a force being exerted on it by the second electrode 8. In this embodiment, the actuation of the first electrode 6 exerts a force on the DUT 3 which is transferred to the force transducer 10 via the DUT 3 and the second electrode 8. The force experienced by the force transducer 10 tends to be indicative of (e.g. is approximately equal to, or is proportional to) the stress applied to the DUT 3 by the first electrode 6 at step s20. In this embodiment, the force measured by the force transducer 10 varies in accordance with the AC drive signal. The force transducer 10 transmits a force signal indicative of the measured force to the differential force amplifier 24. The differential force amplifier 24 amplifies the received force signal and transmits the amplified force signal to the lock-in amplifier module 23.

At step s24, the first lock in amplifier of the lock-in amplifier module 23 processes the received charge signal and the received drive signal, thereby to remove or reduce signal noise and determine a charge value with a signal to noise ratio greater than the signal to noise ratio of the received charge signal. In particular, in this embodiment, the first lock-in amplifier 23 multiplies the drive signal and the received charge signal, and subsequently integrates this product over a time period, thereby determining the charge value, at a frequency set by the drive signal. The time period over which integration is performed may be large relative to the period of the drive signal.

In some embodiments, the first lock-in amplifier 23 determines a phase shift of the charge signal relative to the drive signal. This phase shift may be used, for example by the processor 26, to determine one or more characteristics of the DUT 3, e.g. piezoelectric losses present in real piezoelectric materials.

-17At step s26, the second lock-in amplifier of the lock-in amplifier 23 processes the received amplified force signal and the received drive signal, thereby to remove or reduce signal noise and determine a force value. In particular, in this embodiment, the second lock-in amplifier multiplies the drive signal and the received amplified force signal, and subsequently integrates this product over a time period, thereby determining the force value. The time period over which integration is performed is large relative to the period of the drive signal.

At step s28, the lock-in amplifier module 23 transmits the determined charge value and force value to the processor 26.

At step s30, the processor 26 determines the piezoelectric coefficient of the DUT 3 using the received charge value and the force value.

In this embodiment, the piezoelectric coefficient of the DUT 3 is determined to be:

where: d is the piezoelectric coefficient of the DUT 3;

Q is the charge value; and

F is the force value.

At step s32, the processor 26 sends the determined piezoelectric coefficient to the display 28, and the display 28 displays the piezoelectric coefficient.

Thus, a method of measuring a piezoelectric coefficient of a material is provided.

Advantageously, the first and second electrodes being hemispherical and of large radius of curvature compared to the DUT thickness, tends to produce a substantially linear electric field across the DUT between the electrodes. This tends to improve accuracy of the measured piezoelectric coefficient.

Conventional measurement devices for measuring piezoelectric coefficients tend to use reference piezoelectric materials having known

-18piezoelectric coefficients. However, such conventional devices tend to rely on accurate measurements of the piezoelectric coefficient of the reference material, and also tend to reduce in accuracy over time, e.g. due to degradation of the reference materials. Advantageously, the above-described system and method tend to overcome these deficiencies by avoiding the use of a reference piezoelectric material.

Advantageously, the force transducer tends to be internally calibrated.

The above-described measurement device tends to be especially useful for measuring the piezoelectric coefficient of relatively compliant piezoelectric materials, for example piezoelectric materials that deform relatively easily e.g. when acted on by the first electrode. Typical compliant piezoelectric materials include but are not limited to polymer materials, such as Polyvinylidene fluoride or polyvinylidene difluoride (PVDF). Nevertheless, the above-described measurement device tends to be useable of relative hard, or rigid (e.g. substantially non-deformable) piezoelectric materials.

Relatively compliant piezoelectric materials tend to be susceptible to deformation. Conventionally, this deformation can lead to inaccurate measurements of the piezoelectric coefficients of these materials. Advantageously, the above described system and method tend to address this issue. For example, the above described first electrode may be implemented to determine deformation of a relatively soft material under test, e.g. to measure the indentation depth of an electrode. This can be used to ensure that the deformation of the test material is not sufficient to cause unwanted electrical behaviour of the material, such as stress induced depoling. Also, this can be used to ensure consistent and repeatable deformation of test materials between measurements and between test subjects. For example, force feedback may be implemented to provide for a same initial deformation for different materials being tested. This advantageously tends to provide a constant consistent peak-to-peak AC force.

-19Advantageously, hemispherical electrodes tend to improve predictability of the stresses induced in the material under test compared to, for example, electrodes having flat surfaces and sharp edges.

With relatively soft piezoelectric materials, charge and force signals tend to be relatively very small with respect to background noise components. Advantageously, the above described system and method tend to address this issue. For example, the lock-in amplifier tends to reduce or eliminate the effects of noise.

Advantageously, the piezoelectric disc actuator is held in the casing only at its circumferential edge. The central portion of the piezoelectric disc actuator is free to move to actuate the first electrode. This tends to provide for accurate actuation of the first electrode.

Advantageously, the piezoelectric disc actuator can be selected based on the material under test. For example, the piezoelectric disc actuator may be selected to provide a displacement of the first electrode that avoids damage to the material under test. The piezoelectric disc actuator may be selected to provide a displacement of the first electrode that allows for the full range of piezoelectric response from the material under test.

Advantageously, the above described piezoelectric measurement device may be implemented as a modular system.

Figure 5 is a schematic illustration (not of scale) of a side view of a modular piezoelectric measurement device 46. Elements which are substantially the same as those of Figures 1 to 3 bear identical reference numerals thereto and descriptions thereof will be omitted.

The modular piezoelectric measurement device 46 further comprises a housing comprising an upper jaw 48, a lower jaw 50, and a hinge 52.

The upper jaw 48 comprises a first end and a second end, the second end being opposite to the first end. The upper jaw 48 further comprises a first cavity 54 for receiving the actuation module 4. The upper jaw 48 further comprises a second cavity 56 for receiving the signal processing module 14. The upper jaw

-2048 comprises one or more electrical connections between the first cavity 54 and second cavity 56, to allow electrical signals to be transmitted between the actuation module 4 and the signal processing module 14.

In some embodiments, some or all of the signal processing module 14 is housed within a cavity in the lower jaw 50 instead of or in addition to being housed in the second cavity 56 in the upper jaw 48.

The first electrode 6 is positioned on the lower surface of the first end of the upper jaw 48. The first electrode 6 is operatively coupled to the first cavity 54 such that the actuation module 4 located in the first cavity 54 may actuate the first electrode 6.

The modular piezoelectric measurement device 46 comprises one or more electrical connections between the first electrode 6 and the second cavity 56, to allow electrical signals to be transmitted between the first electrode 6 and the signal processing module 14.

The lower jaw 50 comprises a first end and a second end, the second end being opposite to the first end. The lower jaw 50 further comprises a third cavity 58 for receiving the force transducer 10.

The modular piezoelectric measurement device 46 comprises one or more electrical connections between the third cavity 58 and the second cavity 56, to allow electrical signals to be transmitted between the force transducer 10 and the signal processing module 14.

The second electrode 8 is positioned on the upper surface of the first end of the lower jaw 50. The second electrode 8 is operatively coupled to the third cavity 58 such that the second electrode 8 may apply a force to the force transducer 10.

The modular piezoelectric measurement device 46 comprises one or more electrical connections between the second electrode 8 and the second cavity 56, to allow electrical signals to be transmitted between the second electrode 8 and the signal processing module 14.

-21 The first and second electrodes 6, 8 are positioned such that when the upper and lower jaws 48, 50 are in a closed position (for example, as depicted in Figure 5) the longitudinal axes of the first and second electrodes 6, 8 are substantially aligned.

The upper and lower jaws 48, 50 are coupled together by the hinge 52. The hinge 52 couples together the jaws 48, 50 such that the first end of the upper jaw 48 is located opposite to the first end of the lower jaw 50, and such that the second end of the upper jaw 48 is located opposite to the second end of the lower jaw 50.

The hinge 52 may be, for example, a flexure hinge. The hinge 52 is configured to provide a rotatable coupling between the upper jaw 48 and the lower jaw 50. Thus, by squeezing together the second ends of the upper and lower jaws 48, 50, a user may cause the first ends of the upper and lower jaws 48, 50 (and thereby the electrodes 6, 8) to move apart so as to permit a DUT 3 to be inserted between the electrodes 6, 8. In some embodiments, a different type of coupling between the upper and lower jaws 48, 50 that allows for movement of the electrodes 6, 8 along a straight path is used instead of the hinge 52.

The hinge 52 is further configured to provide a rotational bias to force together first ends of the upper and lower jaws 48, 50. Thus, a user may release the second ends of the upper and lower jaws 48, 50, and the hinge 52 would then move the first ends of the upper and lower jaws 48, 50 together thereby to grip the DUT 3 between the electrodes 6, 8.

With reference to Figure 5, the modular piezoelectric measurement 46 device has a longitudinal length of 120mm, a transverse width of 60mm, and a depth (in or out the page) of 45mm.

The actuation module 4 is removably housed within the first cavity 54. This advantageously tends to allow for removal and replacement of the actuation module 4, for example, to service or maintain the actuation module 4.

The signal processing module 14 is removably housed within the second cavity 56. This advantageously tends to allow for removal and replacement of the

-22 signal processing module 14, for example, to service or maintain the signal processing module 14.

The force transducer 10 is removably housed within the third cavity 58. This advantageously tends to allow for removal and replacement of the force transducer 10, for example, to service or maintain the force transducer 10.

Advantageously, the above described modular piezoelectric measurement device tends to be portable.

Advantageously, the above described measurement devices tend to use low power to operate. The device may be powered by a battery.

Advantageously, the above described system and method provide for fast measurement of a piezoelectric coefficient, for example typically less that one second per measurement.

Advantageously, the measurement device tends to be able to perform multiple tests one after the other, for example, on different parts of the DUT to explore variation across the DUT for quality control.

Apparatus, including the processor, for implementing the above arrangement, and performing the above steps, may be provided by configuring or adapting any suitable apparatus, for example one or more computers or other processing apparatus or processors, and/or providing additional modules. The apparatus may comprise a computer, a network of computers, one or more microcontrollers, or one or more processors, for implementing instructions and using data, including instructions and data in the form of a computer program or plurality of computer programs stored in or on a machine-readable storage medium such as computer memory, a computer disk, ROM, PROM, FRAM etc., or any combination of these or other storage media.

In the above embodiments, the modular piezoelectric measurement device is 120mm x 60mm x 45mm. In other embodiments, the modular piezoelectric measurement device has different dimensions.

In the above embodiments, the first electrode is a banded electrode and second electrode is a non-banded electrode. However, in other embodiments,

-23the first and second electrodes are both banded electrodes. In other embodiments, first and second electrodes are different to each other. For example, the first and second electrodes may comprise different materials to each other. Also, for example, the first and second electrodes may have different shapes and/or sizes to each other, e.g. the radius of the hemisphere of the second electrode may be larger than the radius of the hemisphere of the first electrode.

In the above embodiments, the lock-in amplifier module comprises two lock-in amplifier which determine the charge value and the force value respectively. The lock-in amplifiers may operate in series of in parallel. However, in other embodiments, a different number of lock-in amplifiers is used, for example a single lock-in amplifier may determine both the charge and force values.

In the above embodiments, the first end portion of the first electrode is hemispherical in shape. However, in other embodiments, the first end portion is not hemispherical in shape. For example, the first end portion may be a flat punch, or a relatively sharp/point tipped electrodes.

In the above embodiments, the second end portion of the second electrode is hemispherical in shape. However, in other embodiments, the second end portion is not hemispherical in shape. For example, the second end portion may be a flat punch, or a relatively sharp/point tipped electrodes.

In the above embodiments, the first end portion of the first electrode has a radius of curvature of 5mm. However, in other embodiments, the first end portion has a radius of curvature other than 5mm. Preferably, the radius of curvature of the first end portion is between 5mm and 20mm. Preferably, the radius of curvature of the first end portion is larger than the DUT thickness.

In the above embodiments, the second end portion of the second electrode has a radius of curvature of 5mm. However, in other embodiments, the second end portion has a radius of curvature other than 5mm. Preferably, the radius of curvature of the second end portion is between 5mm and 20mm. Preferably, the radius of curvature of the second end portion is larger than the DUT thickness.

-24In the above embodiments, the actuation axis is a vertical axis. However, in other embodiments, the actuation axis is not a vertical axis.

In the above embodiments, locator rods are used to position the actuation module with respect to the base structure. However, in other embodiments locator rods are omitted. In some embodiments, other locating means such as one or more clamp may be implemented.

In the above embodiments, the AC drive signal is a sinusoidal signal. However, in other embodiments, the AC drive signal is not a sinusoidal signal, for example, the drive signal may be a square or a triangle signal.

In the above embodiments, the AC drive signal has a frequency in the range 5Hz-300Hz. However, in other embodiments the drive signal has a different frequency.

In the above embodiments, the signal processing module comprises a plurality of amplifiers. However, in other embodiments one or more of the amplifiers are omitted.

In the above embodiments, a differential charge amplifier is used. However, in other embodiments, the differential charge amplifier is omitted. In some embodiments, a current to voltage amplifier or a Sawyer Tower charge integrating capacitor may be used instead.

In the above embodiments, the signal processing module is comprised within the measurement device. However, in other embodiments, some or all of the components of the signal processing module are remote from the measurement device.

In the above embodiments, a wave spring is used as biasing means to bias the washer against the piezoelectric disc actuator. However, in other embodiments, one or more different types of biasing means are implemented instead of or in addition to the wave spring.

In the above embodiments, a piezoelectric disc actuator is used to provide actuation of the first electrode. However, in other embodiments, one or more

-25different types of actuator is used to actuate the first electrode instead of or in addition to the piezoelectric disc actuator.

In the above embodiments, the measurement device is arranged to measure the piezoelectric charge generated by the DUT when subject to an 5 external AC cyclic force. However, in other embodiments, the system may be driven differently, such as by a voltage being applied to the DUT and its charge or current measured using the same circuitry to realise a dual function PE loop measurement device.

In the above embodiments, the electrically conductive bands of the first 10 electrode are connected to each other in parallel. However, in other embodiments, the electrically conductive bands are not connected in parallel. For example, in some embodiments, some or all of the electrically conductive bands are electrically isolated from other bands. Some or all of the electrically conductive bands may be separately connected to the charge amplifier or 15 processing module.

Claims

1. A device for measuring piezoelectricity comprising:

a first electrode;

an actuator coupled to the first electrode, wherein the actuator is configured to actuate the first electrode along an axis;

a second electrode positioned along the axis and spaced apart from the first electrode;

a force transducer coupled to the second electrode, wherein the force transducer is configured to output a force signal indicative of a force acting on the second electrode; and wherein the first and second electrodes are configured to output a charge signal indicative of a charge between the first and second electrodes.

2. A device according to claim 1, further comprising a processor configured to determine a piezoelectric coefficient based on the charge signal and the force signal.

3. A device according to claim 1 or 2, wherein at least one of the first and second electrodes comprises a hemispherical portion.

4. A device according to any of claims 1 to 3, further comprising a first amplifier operatively coupled to the first and second electrodes and configured to amplify the charge signal.

5. A device according to any of claims 1 to 4, further comprising a second amplifier operatively coupled to the force transducer and configured to amplify the force signal.

6. A device according to any of claims 1 to 5, further comprising one or more lock-in amplifiers configured to process the charge signal and/or the force signal.

7. A device according to any of claims 1 to 6, further comprising a driver configured to generate a drive signal for actuating the actuator.

8. A device according to claim 7 when dependent on claim 6, wherein the one or more lock-in amplifiers are arranged to receive the drive signal from the driver.

9. A method of measuring a piezoelectric property of a material, the method comprising:

positioning the material between opposing first and second electrode such that the electrodes contact the material;

actuating, by an actuator, the first electrode along an axis, thereby to apply a stress to the material;

outputting a charge signal by the first and second electrodes, wherein the charge signal is indicative of a charge between the first and second electrodes; and outputting a force signal by a force transducer operatively coupled to the second electrode, wherein the force signal is indicative of a force acting on the second electrode.

10. An actuation device comprising:

a casing comprising a cavity, an opening to the cavity, and a flange circumscribing the opening;

a piezoelectric actuator located within the cavity, the piezoelectric actuator comprising a first side and a second side opposite to the first side, wherein the

-28first side of the piezoelectric actuator is coupled to the flange at or proximate to an edge of the first side of the piezoelectric actuator;

a washer located within the cavity, the washer comprising a first side and a second side opposite to the first side, the washer comprising one or more projections extending at least to some extent perpendicularly from the first side of the washer, wherein the one or more projections are coupled to the second side of the piezoelectric actuator at or proximate to an edge of the second side of the piezoelectric actuator; and biasing means comprising a first end and a second end opposite to the first end, the biasing means being coupled between the casing and the second side of the washer, wherein the biasing means is configured to force the washer towards the piezoelectric actuator.

11. An actuation device according to claim 10, wherein the one or more projections extend from the first side of the washer at or proximate to an edge of the first side of the washer.

12. An actuation device according to claim 10 or 11, wherein the biasing means comprises a wave spring.

13. An actuation device according to any of claims 10 to 12, further comprising a mechanical gasket disposed between the piezoelectric actuator and the flange.

14. An actuation device according to any of claims 10 to 13, further comprising an electrode coupled to the first side of the piezoelectric actuator at or proximate to a central portion of the first side of the piezoelectric actuator.

15. An electrode for a device for measuring piezoelectricity, the electrode comprising:

-29an electrically non-conductive body having a convex surface; and at least one electrically conductive band disposed on the convex surface.

16. An electrode according to claim 15, wherein the convex surface is substantially hemispherical.

17. An electrode according to claim 15 or 16, wherein each of the at least one electrically conductive band is centred about an axis of rotational symmetry of the convex surface.

18. An electrode according to any of claims 15 to 17, wherein the electrode comprises a plurality of electrically conductive bands which are spaced apart from each other.

19. An electrode according to claim 18, further comprising an electrical connection configured to connect the electrically conductive bands in parallel.

20. A device comprising:

a housing comprising a first portion and a second portion, the first portion and the second portion being attached together to permit relative movement of the first portion and the second portion;

a first electrode coupled to the first portion;

a second electrode coupled to the second portion and position opposite to the first electrode;

a first cavity located within the first portion and coupled to the first electrode, the first cavity configured to receive an actuation module for actuating the first electrode;

-30a second cavity located within the second portion and coupled to the second electrode, the second cavity configured to receive a force transducer.

21. A device according to claim 20, further comprising an actuation module removably housed in the first cavity and configured to actuate the first electrode.

22. A device according to claim 21, wherein the actuation module is the actuation device of any of claims 10 to 14.

23. A device according to any of claims 20 to 22, further comprising a force transducer removably housed in the second cavity and configured to measure a force acting on the second electrode.

24. A device according to any of claims 20 to 23, further comprising:

a third cavity configured to receive a signal processing module;

a first electrical connection coupled between the first cavity and the third cavity; and a second electrical connection coupled between the second cavity and the third cavity.

25. A device according to claim 24, further comprising a signal processing module removably housed in the third cavity and configured to process electrical signals received via the first and second electrical connections.

26. A device according to any of claims 20 to 25, wherein the first portion and the second portion are hingedly attached.

27. A device according to any of claims 20 to 26, further comprising biasing means configured to bias the first and second electrodes towards each other.

28. A device according to any of claims 20 to 27, wherein at least one of the

5 first and second electrodes is an electrode in accordance with any of claims 15 to 19.

29. A device according to any of claims 1 to 8, wherein the actuator is an actuation device according to any of claims 10 to 14.

30. A device according to any of claims 1 to 8 or claim 29, wherein at least one of the first and second electrodes is an electrode in accordance with any of claims 15 to 19.