TITLE

Piezoelectric device for detecting or generating forces and torques in multiple directions

TECHNICAL FIELD

The present invention relates to a piezoelectric device that is adapted to detect or generate forces and/or torques in multiple directions, in particular, with respect to at least three different degrees of freedom.

The invention further relates to a sensor system comprising such a piezoelectric device, to methods of use of such a device, to a method of self-testing, and to a method of manufacture of such a device.

PRIOR ART

R. Perez et al., "Modeling, Fabrication, and Validation of a High-Performance 2-DoF Piezoactuator for Micromanipulation", IEEE/ASME Transactions on Mechatronics, Vol. 10, No. 2, April 1005, pp. 161-171 discloses a micromanipulator comprising two fingers. Each finger comprises a parallel piezoelectric bimorph having a top and a bottom piezoelectric layer with a central electrode between them. The top and bottom layers are provided with two parallel surface electrodes each. By applying voltages to these electrodes in a particular manner, displacements along two orthogonal spatial directions (i.e., along two degrees of freedom) are generated. The device is limited to two degrees of freedom.

Kui Yao et al., "Design and Fabrication of a High Performance Multilayer Piezoelectric Actuator with Bending Deformation", IEEE Transactions on Ultrasonics, Ferroelectrics, and Frequency Control, Vol. 46, No. 4, July 1999, pp. 1020-1027 discloses a multimorph

actuator with displacement occurring in the transverse, widthwise direction. To this end, pairs of parallel electrodes are provided between a plurality of piezoelectric layers as well as on the outside of the outermost layers. By applying voltages to these electrodes in a particular manner, a transverse displacement in the plane of the piezoelectric layers is obtained. The device is limited to a single degree of freedom.

US 4,802,371 discloses a multi-component dynamometer for measuring forces and torques in different directions. The dynamometer comprises two force induction plates and a plurality of independent transducer elements arranged between them. Each transducer element comprises a rectangular carrier plate and two pre-oriented piezoelectric elements fixed thereon, one of these being sensitive to shear in a direction parallel to one edge of the carrier plate and the other one being sensitive to pressure in a direction perpendicular to the carrier plate. While this device allows for a determination of forces and torques in more than two directions, the device is relatively complex, bulky and difficult to manufacture.

US 5,682,000 discloses a piezoelectric sensor comprising a flexible substrate. A single piezoelectric layer is disposed on top of the substrate and is provided with a plurality of electrodes on both sides. A working body serves to transmit forces to the substrate. Upon application of a force to the working body, the substrate and the piezoelectric layer are deformed, leading to charge accumulation at the electrodes. The charge pattern at the electrodes depends in a characteristic manner on the direction of the applied force. Thereby, directional information about the applied force is obtained. However, also this device is relatively complicated.

WO 2007/054781 discloses a combined sensor and actuator based on a single piezoelectric bimorph. No directional information is obtained by the sensor, and the actuator can provide a force only along a single direction.

US 6,681,631 discloses a piezoelectric sensor comprising a single piezoelectric layer provided with electrically conductive contact layers on both sides. In one embodiment, the contact layers are subdivided into segments in order to improve sensitivity and to obtain redundancy. No directional information is obtained.

US 5,797,623 discloses a piezoelectric impact sensor. A single piezoelectric layer is disposed between two conductive layers. In one embodiment, one of these layers is rectangular and divided along its diagonal in order to obtain information about a point of impact on the sensor. No directional information is obtained.

SUMMARY OF THE INVENTION

There is therefore a need for a simple, robust and cheap piezoelectric device that is adapted to determining or generating loads selected from forces and torques associated with more than two degrees of freedom.

This need is addressed by a piezoelectric device as defined in claim 1. Thus, the invention provides a piezoelectric device comprising a first piezoelectric layer, the first piezoelectric layer having a first side and a second side and defining a device plane; a second piezoelectric layer disposed parallel to and overlapping with the first piezoelectric layer, the second piezoelectric layer having a first side and a second side, the first side of the second piezoelectric layer facing the second side of the first piezoelectric layer; a reference electrode disposed between the second side of the first piezoelectric layer and the first side of the second piezoelectric layer; at least three outer electrodes being disposed on the first side of the first piezoelectric layer and at least one outer electrode is disposed on the second side of the second piezoelectric layer.

In other words, the piezoelectric device of the present invention comprises a piezoelectric bimorph having at least four outer electrodes distributed over its top and bottom sides, at least three of these electrodes being disposed on one side of the bimorph, and at least one electrode being disposed on the other side of the bimorph.

In particular, the outer electrodes are distributed in a manner that an application of a load selected from forces and torques to the device, the load being associated with any of at least three different degrees of freedom, causes a voltage distribution on said outer

electrodes that allows to determine the degree of freedom that said load is associated with. This will also allow for a generation of loads with respect to these at least three degrees of freedom if the device is used as an actuator.

Here, the term "degree of freedom" is to be understood as normally used in the field of mechanics. In particular, the degrees of freedom (DOF) of a rigid body are defined as the set of independent translations and/or rotations that specify completely any displaced position and orientation of the body. A rigid body generally has six DOFs: three translational DOFs and three rotational DOFs. A change in the state of a body with respect to a translational DOF is associated with a force in a certain direction, a change in the state of a body with respect to a rotational DOF is associated with a torque (moment) around a certain direction.

It is to be understood that all electrodes are electrically conductive and in contact with the piezoelectric layer(s) on which they are disposed. In particular, the reference electrode is in electric contact with both the first (upper) and second (lower) piezoelectric layer. The electrodes may be manufactured from any electrically conductive material, such as metals or conductive polymers, and may be attached to the piezoelectric layers by any known method, such as vapor deposition techniques or bonding of a foil by any suitable method.

The piezoelectric layers are generally flat and plate-like. In the following, they are also designated as piezoelectric plates. These layers may consist of any material exhibiting a sufficiently strong piezoelectric effect, in particular a ceramic material such as PZT (lead zirconium titanate) or a polymer material such as PVDF (polyvinylidene fluoride). PZT plates are preferred.

Directions are identified as follows: The device plane is designated as the X-Y plane. The direction along the normal of the device plane is designated the Z direction. If the piezoelectric layers have an elongated shape (e.g., a rectangular, but not square, shape), the direction parallel to the long axis is designated the X direction. The Y direction is perpendicular to both the X and Z directions, the three directions defining a right-handed coordinate system.

Many different distributions of the electrodes are possible in order to be able to separate loads associated with at least three DOFs, and no simple general definition can be given. Examples will become apparent from the detailed description of preferred embodiments below.

In one specific embodiment, the device comprises eight outer electrodes, a first set of four of said outer electrodes being disposed on the top side of the first piezoelectric layer, and a second set of four of said outer electrodes being disposed on the bottom side of the second piezoelectric layer. The electrodes of each set may advantageously be arranged in identical quadrilateral arrangements, such that each outer electrode from the first set faces an outer electrode from the second set across said piezoelectric layers. In general, this distribution of electrodes ensures that forces and torques along five degrees of freedom may be distinguished. In particular, if the sensor is cantilevered at one of its ends along the X direction, forces in the X, Y and Z direction and torques around the Y and Z directions may be distinguished, as will become apparent from the detailed description of preferred embodiments below.

In such embodiments, the determination of the load is simplified if all outer electrodes have substantially the same surface area. However, this is by no means necessary, and it is well conceivable to have a different size distribution of the electrodes.

In a preferred embodiment, the piezoelectric layers are of identical elongated shape defining a longitudinal direction, and each outer electrode has an elongated shape defining a longitudinal direction substantially parallel to the longitudinal direction of the piezoelectric layers. In particular, the piezoelectric layers may be rectangular and identical in size, thus overlapping completely, and the electrodes may also be rectangular, the long axis of the electrodes being parallel top to the long axis of the piezoelectric layers.

Preferably, the bimorph is an antiparallel bimorph, i.e, the first piezoelectric layer has a first polarization direction perpendicular to the device plane, and the second piezoelectric layer has a second polarization direction that is antiparallel to the first polarization direction.

In order to protect the piezoelectric layers and the electrodes, the device may comprise a polymer coating and/or a polymer casing in which these components are embedded.

In one embodiment, the device comprises an electric connector and a rod, the piezoelectric layers and the electrodes being disposed at a first end of said rod, the connector being disposed at a second end of said rod, and the electrodes being electrically connected to contacts of said connector.

In another embodiment, the piezoelectric layers have an elongated shape defining a longitudinal direction, and the device comprises a first force induction element disposed near a first end of said piezoelectric layers and a second force induction element disposed near a second end of said piezoelectric layers. In particular, the force induction elements may be disk-shaped, the disk axis coinciding with the long axis of the piezoelectric layers.

The piezoelectric device is particularly well-suited for use of a sensor. In this regard, the invention also provides a piezoelectric sensor system comprising: a piezoelectric device as described above; and an electronic evaluation unit, the evaluation unit being electrically connected to said reference electrode and to said outer electrodes, the evaluation unit being specifically adapted to receive a plurality of input values corresponding to potential differences between said outer electrodes and said reference electrode and to determine from said input values at least three output values, each output value representing a load selected from forces and torques, the load being associated with a predetermined degree of freedom, the degrees of freedom being mutually different.

In another aspect, the invention provides a method of use of a piezoelectric device as described above for determining forces and/or torques associated with at least three different degrees of freedom. In other words, the invention provides a method comprising the steps of providing a piezoelectric device as defined above; applying at least one load selected from forces and torques to the device; measuring a voltage distribution of voltages between the outer electrodes and the reference electrode; and

from said voltage distribution, determining a measure of the magnitude of the load for each of at least three predetermined degrees of freedom.

In yet another aspect, the invention provides a method of use of a piezoelectric device as described above for generating forces and/or torques for at least three different degrees of freedom. In other words, the invention provides a method comprising the steps of providing a piezoelectric device as defined above; and applying a voltage distribution to said outer electrodes and said reference electrode to cause the piezoelectric device to generate forces and/or torques associated with at least three predetermined degrees of freedom.

In still another aspect, the invention relates to a method of self-testing of a piezoelectric device as described above. The method comprises: applying an excitation voltage between at least one of said outer electrodes and said reference electrode to excite a movement of the piezoelectric device; measuring a response voltage between at least one of said outer electrodes and said reference electrode in response to said movement; and determining whether the response voltage is within a predetermined expected range.

The response voltage may be measured simultaneously with application of the excitation voltage. In this case, the outer electrodes to which an excitation voltage is applied are different from the outer electrodes at which the response voltage is measured. However, it is also possible to measure the response voltage after application of the excitation voltage, for the same of for different outer electrodes than those to which the excitation voltage was applied.

In another aspect, the invention provides a method of manufacturing a plurality of piezoelectric devices in parallel. The method comprises providing a piezoelectric bimorph comprising a first piezoelectric layer, a second piezoelectric layer, a central layer of an electrically conducting material disposed between said first layer and said second layer, and a first and a second outer layer of an electrically conducting material disposed on the sides of the first and the second piezoelectric layers

facing away from the central layer; patterning said first and second outer layer to obtain a plurality of outer electrodes for a plurality of piezoelectric devices; after said step of patterning, cutting said bimorph into separate pieces to obtain the plurality of piezoelectric devices; and after said step of cutting, providing connecting wires to said electrodes by soldering or bonding.

BRIEF DESCRIPTION OF THE DRAWINGS

Preferred embodiments of the invention are described in the following with reference to the drawings, which show:

Fig. 1 a perspective view of a piezoelectric device embedded in a casing; Fig. 2 a perspective view of the device of Fig. 1 without the casing;

Fig. 3 a different perspective view of the device without the casing;

Fig. 4 an illustration of selected strain profiles in the device of Fig. 1 ;

Fig. 5 an illustration of a method of manufacture for the device of Fig. 1;

Fig. 6 an illustration of an alternative electrode configuration; Fig. 7 a side view of a sensor system, partially in sectional view;

Fig. 8 an example of a sensor device, shown in a partial sectional view through plane C of Fig. 9;

Fig. 9 a plan view of the sensor device of Fig. 8 in direction B of Fig. 8;

Fig. 10 the sensor device of Fig. 8, partially in sectional view in plane D of Fig. 9; and

Fig. 11 a plan view of the sensor device of Fig. 8 in direction A of Fig. 8..

DESCRIPTION OF PREFERRED EMBODIMENTS

In Figs. 1 to 3, a piezoelectric device particularly suited as a sensor is shown. Figures 1 and 2 illustrate the main components of the sensor. The device comprises a piezoelectric bimorph 1 that is able to sense external force and torque applied on it from any direction, i.e. a five-DOF (Degrees of Freedom) force/torque sensor. The sensing elements are the

two piezoelectric plates 3, 4 of the bimorph. The outer electrode layers 6, 7 of the bimorph are patterned in order to create eight separate electrodes, and to each electrode an output wire 2 is connected (connections 8). The sensor is packaged in a casing 10, or it is embedded in a flexible layer that protects it.

The sensor's working principle is piezoelectric. Force or torque applied on the piezoelectric bimorph creates axial and/or bending stresses (depending on the direction of the external load). Strain is created in the beam and converted by the direct piezoelectric effect into electric displacement. Change in electric displacement accumulates charge on the sensors electrodes which is measured by external instruments such as a voltmeter. Figure 3 is an example of the electrode configuration in the bimorph. The top and bottom electrode layers 6, 7 are divided into four equal segments S1-S4 and S5-S8, respectively, and the middle electrode 5 is used as the common reference voltage.

The different output signals from the sensor are designated by S1-S8. Sl, S2, S3 and S4 are the signals of the electrodes closer to the base of the sensor in clockwise ascending order. S5, S6, S7 and S8 are the signals of the electrodes near the tip of the sensor.

Figure 4 illustrates the motion of the bimorph and the strain profiles 1 1, 12 and 13 created due to loads in the -X, -Y and Z directions, respectively.

Applying a force Fx in the X direction creates uniform compressive or tensile stress, and as a result a uniform axial strain (see Fig. 4) in the top and bottom piezoelectric plates. Assuming the polarization in the bimorph is in an anti-parallel configuration, a symmetric charge (the plane of symmetry being the X-Y plane) will be created in the different electrodes, see Table 1. However, a force normal to the X axis (Y or Z direction) will create a bending stress and as a result an anti symmetric strain profile (the plane of the anti-symmetry being X-Y or X-Z when Fz or Fy are applied accordingly) in the beam (see Fig. 4). The strain anti-symmetric charge due to bending can be easily separated in the different electrodes by a simple logical circuit.

A moment around the X axis, Mx, will twist the beam and create a symmetrical shear stress around the X axis. The piezoelectric effect converts such a stress into charge in the Y

direction. Since there are no electrodes to collect the charge in order to simplify the manufacturing of the sensing element, this charge cannot be sensed directly.

A moment orthogonal to the X axis (Y or Z direction) will create a bending stress similarly to a force. Since the bending moment My and Mz create a constant strain along the beam the charge on the electrodes near the base of the sensor will be equal to the charge at the electrodes near the tip of the sensor. The forces Fz or Fy will not create a similar charge on the electrodes because the bending moment created by the force increases linearly from the tip to the base of the beam. By comparing the electrodes S1-S4 to the electrodes S5-S8, accordingly one can separate between the force and the moment applied in the Y and Z directions (see Table 1).

Table 1: Comparison of the charges created by the different loads applied on the sensor. Ql, Q2 and Q3 designate mutually different charges of arbitrary magnitude.

The sensor output will be connected to an amplifier (depending of the force range needed to be measured) and an eight-channel Data Acquisition Card.

In order to illustrate the sensors potential a numerical example of the sensors force sensing capability is given in the following:

Let us assume that the sensor length is 1 mm, the thickness of each piezoelectric layer is

0.05 mm (the thickness of the electrodes are neglected), the width is also 0.1 mm, and the beam is made of PZT. When the force is in Z direction the stress that is exerted in the beam

is:

Eq. (1) σ = -

A where F is the exerted force, A is the cross section area of the sensor.

The electric displacement is:

Eq- (2) D = ^f, where d _3i is the piezoelectric coefficient and A is the cross section area of the beam.

The charge that is created in each PZT layer is: where A _c is the area of the ferroelectric capacitor. L and t are the length and thickness of each PZT element.

The voltage created in each ferroelectric capacitor is: d, _λ FA _c Eq. (4) V = ^- = — ^- = ^L ,

C ^A_ ε _3i b t where t and b are the thickness and width of each piezoelectric layer and ε ₃₃ is the dielectric constant of the PZT.

Substituting the PZT properties, see Elad Tadmor, Gabor Kόsa, "Electromechanical coupling correction for piezoelectric layered beams", Journal of Micro Electro Mechanical Systems, 12(6), 899-906, December 2003, and the geometrical dimensions into Eq. (4) gives the following numerical values:

—171 -1O ^-12

Eq. (5) F[VoItSl = -—^ _r F[Nl = 1 13.6F[N] .

15.05 - 10- ⁹ I lO- ^{4 L}

A sensor of the size of 0.1 mm x 0.1 mm x 1.0 mm is able to provide 1.1 Volts for every gram of mass that is applied on it without any amplifier.

The capacitance of the ferroelectric capacitor is: c _{= jj} A ₌ 15.05 -10-M .10- - 1.10- ³ _{pF = 30 pF}

Eq. (6) t 0.5 -10

Using for example a PCI-DAS6014 DAQ card by Measurement Computing (an equivalent of NI's PCI-6014 card) with internal impedance of 100 GOhm the time constant of the RC circuit created by the sensor and the measurement system is:

Eq. (7) r = /?C = 100 -10 ⁹ -30 -10 ^"12 s = 3 s

The measurement systems functions as high pass filter and the time constant designates the knee frequency, i.e. the sensor is able to measure signals above 1 Hz. The higher frequency is limited by the mechanical natural frequency of the beam and it is above 1 kHz.

One can overcome the DC voltage measurement limitations by the proper electric circuitry, if it is necessary.

A preferred manufacturing method of the sensor will be parallel processing of a piezoelectric bimorph plate. First the upper and lower electrodes will be patterned, second the sensing elements will be diced by a standard semiconductor dicing saw, and then the connections 8 will be soldered or bonded to the electrodes. The sensor may be packaged by polymer coating 9 or in a pre-manufactured polymer casing, or it may be embedded in a larger pattern to create a sensor array.

Figure 5 illustrates these steps of the manufacturing process of the force sensor: a) Raw material: piezoelectric bimorph plate, b) Patterning by etching, laser ablation or sand blasting of the top electrodes, c) Patterning by etching, laser ablation or sand blasting of the bottom electrodes, d) Dicing, e) Bonding of contact wires 2 and f) packaging by coating with a polymer 9 such as parylen C.

Figure 6 illustrates an alternative electrode arrangement. On the top side of the bimorph, three outer electrodes Sl', S2' and S3' are disposed, shown in solid lines, while on the bottom side, one single electrode S4' is disposed, shown in broken lines. The reference

electrode between the two piezoelectric layers of the bimorph is not shown in this schematic illustration. Such a device will allow for separation of at least three degrees of freedom. In the particular arrangement of Fig. 6, loads associated with four degrees of freedom can be separated: forces in the X, Y and Z direction and torque around the Y axis. It is readily apparent from this exemplary illustration that an infinite number of other electrode arrangements will be possible.

Fig. 7 illustrates an example of a sensor system particularly well suited for measurements in locations that are difficult to reach. A bimorph 1 as shown in Figs. 1 to 3 is mounted with one end to a long, narrow rod 22 that ends at its opposite end in an electric connector 20. The output wires 2 are connected to this flat connector, having nine contacts 21 thus connected to the reference electrode 5 and to the eight outer electrodes S1-S8. The connector connects to an electronic evaluation 23 unit that measures the voltages at the contacts 21 and determines, from the voltage distribution, the values of forces Fx, Fy, and Fz and of torques My and Mz by applying the voltage-load relationship of Table 1. To this end, the evaluation unit comprises an analog-to-digital converter (ADC) and a processor, the ADC receiving input values 24 in the form of voltages, converting the voltages into digital voltage values, and the processor determining, from the digital voltage values, output values 25 representing said forces and torques. These output values may be displayed on a monitor screen, written to a memory for logging, or may be further processed in any conceivable way. Numerical methods for "inverting" the matrix of Table 1 to obtain the output values from the input values are well known in the art.

For performing a self test, the evaluation unit may comprise a voltage source or an oscillator for exciting a vibration of the bimorph in a self-testing mode. In this mode, the voltage source or oscillator provides excitation voltages to at least one or all of the outer electrodes, e.g., for exciting a thickness mode vibration of the bimorph. After the excitation voltages have been switched off, the thickness mode is detected by measuring voltages at the electrodes and comparing these to the expected voltages. Any differences exceeding a predetermined threshold will then indicate a failure of the sensor. Other modes of operation of the self-test facility are of course possible, such as simultaneous excitation at some of the electrodes and detection at other electrodes.

It is to be understood that the evaluation unit may also be constructed in a different way, e.g. in purely analog circuitry or by employing a general -purpose computer equipped with a data acquisition card, and that the use of such a unit is not limited to the piezoelectric device shown in Fig. 7.

Figures 8 to 11 illustrate another embodiment of a sensor 30 employing the bimorph 1 of Figures 1 to 3. In this embodiment, force induction elements are mounted to the ends of the bimorph 1 with the aid of an adhesive mass 31, 32. Each force induction element is composed of two concentric disks 33, 34 and 35, 36, respectively, of identical diameter, i.e., each element is disk-shaped, the disk axis coinciding with the long axis of the bimorph (the X axis). The wires 2 are guided through the space between these disks and are embedded in the adhesive mass. The force induction elements serve to transfer forces and moments to the bimorph and aid the user in mounting the device on some base. A cylindrical tube made of a relatively soft, pliable and compressible material such as silicone rubber serves as a protective sleeve 37 in order to protect the electrodes of the bimorph. It is to be noted that the views of Figures 8 to 11 are only schematic. Figs. 8 and 10 show schematic sections through two mutually orthogonal sectional planes C and D; however, some parts such as the wires 2 are still shown for the sake of clarity despite being situated behind the sectional plane. Figs. 9 and 11 show schematic plan views of the sensor in which the outermost disks 34 and 36, respectively, have been removed.

The devices described above may also be used to generate forces and /or torques associated with different degrees of freedom, instead of measuring such loads, simply by applying appropriate voltages to the electrodes. In particular, for the electrode arrangement of Fig. 3, voltages as given in Table 1 may be applied to generate forces Fx, Fy, and Fz and/or torques My and Mz.

From the foregoing examples, it will be apparent that many modifications are possible without leaving the scope of the invention, and the invention shall not be construed as limited by these examples.

LIST OF REFERENCE SIGNS

bimorph 22 rod wire 23 evaluation unit first (upper) piezoelectric plate 24 input values second (lower) piezoelectric plate 25 output values reference electrode 30 sensor top electrode layer 31, 32 adhesive mass bottom electrode layer 33-36 disks wire connection 37 tube polymer A, B view direction casing C, D sectional plane strain profile S1-S8, S1 -S4' electrodes connector Fx, Fy ₅ , Fz forces contacts My, Mz torques