Piezoelectric Foil Sensors and Actuators
Piezoelectric films made of PVDF (Polyvinylidene fluoride) are very thin and flexible active electromechanical transducers. They can convert about 12% of mechanical energy into electrical energy, and vice versa. Thus they are useful as a sensor as well as an actuator.
The piezo foil technique allows the design of extremely flexible and thin sensors as well as actuators, which may be placed on 3D-contoured surfaces or inserted in small gaps as well as embedded in GRP/CRP structures.
The piezofilm is a transparent film of highly polar poly-vinylidene-fluoride (PVDF). In proportion to the influence of mechanical stress or strain the film develops an electrical charge that could be shunted over the metal-coated surfaces. Otherwise the application of electric current leads to a proportional mechanic deformation.
The direct piezoelectric effect is used in sensor applications and the inverse piezoelectric effect in actuator applications.
In electronic terms the piezofilm is a capacitor that reacts with a change in electric charge proportional to the change of applied force. Piezoelectric materials are anisotropic. This means, that their electrical and mechanical properties vary with the direction of applied force, strain or electrical field.
The film´s axes are identified by the numerals indicated in fig. 1. The symbols listed in Technical Data for strain- and stress constant dii and gii are declared as follows. The first index identifies the direction of applied electrical field (always the direction of the thickness of the film when using piezoelectric films). The second index refers to the axis of induced mechanical strain or applied stress.
Innovations & Advantages
Due to the piezo-active measurement principle only very small contact wires are required, allowing the integration of a high number of sensors with a spatial resolution down to sub-mm scales. Due to anisotropic properties of mono-axial oriented foils the charge produced by different strain components is amplified or dampened, resulting in a multitude of specific sensor applications. Several multi-layer and multi-element actuators can be realized, producing longitudinal or torsional motion or traveling waves in beam elements.
Current and Potential Domains of Application
The sensor foils are suitable for applications in the areas of dynamic force, pressure, motion and temperature measurements up to high frequencies. Some realized examples are: fluid, IR, vibration and pulse pressure sensors, keyboards, robotics, tactile and ultrasonic sensors. Piezoelectric actuators are suitable for large displacements with small forces as for example the damping or amplification of oscillations of lightweight structures, optical components and miniature positioning devices or fans.