*4.2.2. d*<sup>33</sup> *effect actuators*

Piezoceramic actuators using the *d*<sup>33</sup> effect are based on the fact that a through-thickness electrical field will modify the material's thickness. Within piezoceramics, the *d*<sup>33</sup> coefficient is always more important than the *d*<sup>31</sup> coefficient, therefore using the *d*<sup>33</sup> effect is preferable. Consequently, there are many types of actuators that try to take advantage of the larger *d*<sup>33</sup> coefficient using various geometries.

• Stack actuator

Stack actuators use the *d*<sup>33</sup> effect to achieve deflection. They consist of multiple layers of piezoceramic plates separated by electrodes as shown in Figure 12. This configuration allows long elements to be made for higher displacement capabilities while high voltage is not needed to obtain high electrical fields if the piezoceramic layers are small enough between two electrodes. Stack actuators are capable of delivering higher forces than laminar actuators as they are fully using the highest strain coefficient available.

• Macro Fibre Composites (MFC) & Active Fibre Composite (AFC)

Long piezoceramic components have better displacement capabilities. Active Fibre Composite (AFC) consists of piezo ceramic fibres embedded into a protective polymer substrate and poled into the fibre direction to use the highest strain coefficient. Interdigitated electrodes bonded onto the fibres ensure high electrical fields. The voltage required by these components depends on the fibre diameter and the distance between electrodes. Compared with laminar actuators, these components require less voltage to achieve the same force and displacement. Macro Fibre Composite exploit the same principles as AFC, except that they are made of fibres having an improved contact with electrodes, which plays an important role in the electrical field magnitude inside the material. Furthermore, those actuators are much more flexible than a strip made of the same material.

#### 670 Smart Actuation and Sensing Systems – Recent Advances and Future Challenges Smart Actuation for Helicopter Rotorblades <sup>15</sup> Smart Actuation for Helicopter Rotorblades 671

**Figure 12.** Principle of a piezoceramic stack actuator.

**Figure 13.** Sketch of an Active Fibre Composite (AFC) actuator.
