State-of-the-art on dynamic modeling of MFC transducers
2.2 Advances of piezoelectric transducers
The piezoelectric constantse are the coupling coefficients between stresses and electrical field in the corresponding direction.
2.2 Advances of piezoelectric transducers
A simple piezoelectric transducer consists of a thin polarized piezo-patch with a pair of electrodes on the top and bottom surface of the patch. The thin piezo-patch can only have one work mode in the Figure 2.2 according to its poling direction. Let’s take the d31/d32 work mode as an example. It can only perform in-plane expansion/contraction when an operational voltage is
ADVANCES OF PIEZOELECTRIC TRANSDUCERS 13
applied to it. However, the piezo-patch can be easily integrated into a host structure, as shown in Figure 2.3. When it functions as an actuator, the asymmetric deformation in the thickness-wise of the structure results in both the local contraction/extension and bending at the same time. Conversely, the deformation of the piezo-patch will lead to a direct piezoelectric effect on the transducer too.
In-plane Out-plane
Piezoelectric transducer
Figure 2.3: Work principle of a thin piezoelectric transducer
Based on the work mode of the thin piezo-patch, different piezoelectric actuators have been designed to generate different performances, such as piezo-stack actuators and piezo-benders, as shown in Figure 2.4. They consist of multilayer piezoelectric patches with electrodes in each other. The small distance between two neighboring electrodes can generate a large electric field with for a given operational voltage. On the piezo-stack actuators, all the patches deform in the same way under an operational voltage. Hence, they can generate large actuation force/deformation. The piezo-bender in Figure 2.4is composed by two piezoelectric layers. The difference in deformation between the two layers leads to bending deformations.
0
V 0 V V
+
V
-V P
P PP
PP PP PP PP
PP PP PP
(a) piezo stack actuator (b) piezo shear actuator (c) piezo bender
Figure 2.4: Multilayer piezoelectric actuators (The red and blue dash-lines represent the deformations of the actuators)
All the mentioned piezoelectric transducers/actuators are made of monolithic piezoelectric layers and their high stiffness affords a large actuation force and a voltage-dependent actuation authority [40]. However, the limitations of monolithic piezoelectric transducers are pronounced too [14,41]. The brittle nature makes them damage easily. Moreover, the high mass density and high stiffness may severely modify the dynamics of lightweight, flexible host structures.
last but not the least, the limited mechanical flexibility makes them difficult to adapt non-flat host structures too.
Composite piezoelectric transducers have greatly overcome these limitations.
The main idea of the composite transducers is to embed the piezoelectric fibers into an epoxy matrix so that the hybrid transducers have high performance, good flexibility, and high durability. Various advanced piezoelectric transducers have already been commercialized, such as MFC and Active Fiber Composite (AFC)[14, 41]. The MFC and AFC transducers share the same mechanism.
But the rectangular piezoelectric fibers of MFC transducers lead to a higher fiber volume fraction than AFC, which uses the round piezoelectric fibers.
As a result, the performance of MFC transducers is almost 1.5 times larger than AFC transducers. The experimental results also proved that the actuation performance of MFC is better than many other piezoelectric actuators after more than 90 million electrical cycles [14, 42]. Besides, the rectangular piezoelectric fibers make MFC transducers easier and less expensive to manufacture than AFC transducers. Hence, the MFC transducer is one of the most promising choices for engineering application. There are different kinds of MFC transducers:
MFC-d31, MFC-d33 and MFC-d15, as shown in Figure 3.4-2.8. The MFC transducers commonly have a rectangular geometry. And they usually consist of five layers: two Kapton layers, two electrode layers, and one active layer. In the following schematic representation figures from [43], the acrylic layers are considered as an individual layer. Besides, it is worthwhile to notice that the MFC transducers produced by the different manufacturer could have different kinds of electrodes according to their production techniques.
The MFC-d31 transducer has a poling direction in the thickness-wise of the transducer. A pair of continuous electrodes is used in the transducer in the figure so that the electric field is uniform in the thickness-wise of the transducer.
Thereby, both thed31 andd32 effects in theX−Y plan can be used for the sensing/actuation. When it is used as an actuator on a host structure, it can generate both expansion/contraction and bending motions in bothX andY directions.
ADVANCES OF PIEZOELECTRIC TRANSDUCERS 15
Figure 2.5: Schematic representation of MFC-d31 [43] ((X, Y, Z) represents the material coordinates system and (x1, x2, x3) denote the structural coordinates system)
The MFC-d33 transducer has a complex poling direction along with the piezoelectric fiber according to the neighboring positive and negative electrodes fingers. The d33 effect, which is usually much larger than d31 and d32
has a unidirectional sensing/actuation. It can generate a unidirectional expansion/contraction and bending behaviors on a host structure when it works as an actuator. The performance of the transducer increases with the distance increasing between the adjacent electrode fingers, and a high operational voltage is required to generate the necessary electric field. That is due to the fact that the electrodes generate a non-uniform electric field, as shown in Figure 2.7. A larger distance between the adjacent electrode fingers can reduce the dead zone and generate a more uniform electric field in the piezoelectric fibers.
Figure 2.6: Schematic representation of MFC-d33 [43] ((X, Y, Z) represents the material coordinates system and (x1, x2, x3) denote the structural coordinates system)
electrodes
Piezoelectric fiber section Electric field