Piezo Transducer Tutorial: Patch Transducers

Working Principle
The term piezo is derived from the Greek word for pressure. In 1880 Jacques and Pierre Curie discovered that an electric potential could be generated by applying pressure to quartz crystals; they named this phenomenon the piezoelectric effect. Later they ascertained that when exposed to an electric potential, piezoelectric materials change shape. This they named the inverse piezoelectric effect.

The piezo effect is used for sensor functionality, while actuator behavior uses the inverse piezo effect.

The piezoceramic plates in DuraAct™ patch transducers resemble a capacitor. The ceramic acts as a dielectric between its metallized surfaces. When voltage is applied, an electric field is created inside the ceramic. The field causes a uniform lateral contraction of the ceramic perpendicular to the direction of the electric field (Fig. 1). This behavior is called the transverse piezoelectric effect (d31 effect, Fig.2).

The electric field strength determines the magnitude of the lateral contraction. This is the key to simple control of the transducer modules. When the modules are glued to a substrate, they effectively transfer force over the whole surface, not only at selected points, as with conventional actuators. Conversely, DuraAct™ patch transducers transform changes in shape to electric current, thereby enabling their use as sensors or energy sources.

The piezoceramic response to a change of the electric field or to deformation is extremely fast. Vibrations in the kilohertz range can be produced or detected. Different excitation voltages are required and different contraction amounts possible, depending on the ceramic type and its dimensions. The correlation between displacement and applied voltage is not linear. A voltage-to-displacement curve with the typical hysteresis behavior is shown in Fig. 3.

Technology
DuraAct™ piezo transducers operate as sensors with varying bandwidths—reacting to mechanical strain like impact, bending or pressure—and as high-precision positioning or bending actuators.

The standard transducerdesign features a piezoceramic foil with metalized surfaces for electrical contact (Fig. 4). The thickness of standard foils used is typically 100 to 500 ìm, with even thinner layers possible. Without further processing, these piezoceramic elements are brittle and difficult to handle. Embedding them in a polymer structure provides electrical insulation and mechanical stability. The result is a module that is ductile and extremely robust.

An alternative design features multiple layer piezoceramics, enhancing force generation for the same operating voltage.

DuraAct™ piezoelectric transducers are solid state actuators and therefore have no moving parts. Wear and failure rates are low. Electrical contact is realized by soldering, clamping or gluing leads to two pads. Connecting multiple layers separately allows separation of the sensor and actuator functionality, meaning that the transducer can be used as sensor and actuator simultaneously.

Working Diagram
The actuator properties of piezoceramic transducers are essentially described by two parameters: the blocking force FB and the free displacement, S0. When a voltage U is applied to the free (unblocked) actuator,it reaches its maximum displacement S0. The force required to prevent any length change at all is called the blocking force, FB (Fig. 5).

A graph of applied force versus actuator displacement is called the actuator characteristic curve (Fig. 7). It basically follows the line passing through the points with 0 force and 0 displacement described above. In most cases the actuator acts against an elastic structure, e.g. when a spring or a metal sheet is deformed (Fig. 6). If the load is represented by a spring (characteristic curve of the spring) with stiffness of cF, the resulting operating point is the intersection of the load line with the actuator characteristic curve (Fig. 7). The most effective operation occurs when the operating point is in the middle of the characteristic curve.

Parameters for Bender Actuators
DuraAct™ actuators are usually glued to a substrate and transfer the contraction not at a few attachment points, but over the whole surface. In such a configuration, the DuraAct™/ substrate combination acts as a bender actuator. Bender actuators provide fast, high-precision and repeatable deflection and are used in a wide range of applications, e.g. in printers, valves, and in the textile industry. DuraAct™ patch transducers are based on the transverse piezo effect, and therefore contract with an electric field applied. The bender flexes and exerts a normal force as shown in Fig. 8. For the free, unblocked bender, the free deflection is W0. The force required to reduce the deflection to zero is called the bender blocking force FBW. It is significantly smaller than the actuator blocking force. The line through these two points, gives the characteristic curve for the bender. Fig. 9 and 10 show curves relating the maximum deflection W0 and the maximum force FBW to the substrate thickness and elasticity. These diagrams show the actual deflections and forces measured with 50 mm substrate samples made of different materials and a P-876.A15 DuraAct™ patch transducer. Together with the characteristic curve for the DuraAct™ alone, the bender characteristics form the basis for effectively estimating the actuator performance in a specific application. PI therefore includes these curves on all datasheets.

Power Requirements
To determine the required electrical power for successful actuator operation, the electrical capacitance must be known. Typical DuraAct™ capacitances are in the nanofarad range and can be found in the datasheets. The electrical capacitance, C, depends on the piezoceramic type, thickness and area. For an estimation of the average electrical power, Pm, knowledge of the operating voltage range and the excitation frequency is necessary.

f: Frequency
Uh: Voltage swing

The maximum power required (Pmax)
is then just the average power times pi (p):