DuraAct Patch Transducer Technology

Manufacture

The active layer of DuraAct patch transducers consists of a piezoelectric plate. These plates are embedded in fiber-reinforced plastic (GRP) according to a patented process and the bonded to form a composite. The lamination process is done in an autoclave in a vacuum, using an injection method. This results in completely bubble-free laminates of the highest quality.

The curing temperature profile of the autoclave is selected so that a defined internal preload of the piezoceramic plates will result due to the different thermal expansion coefficients of the materials involved. The polymer coating of the GRP also serves as electrical insulation and as mechanical preload. Robust, bendable transducer elements that can be manufactured in large batches are the result of this patented technology.


All-Ceramic or Multilayer Patch Transducers

Depending on the active height, all-ceramic piezoelectric contracting plates are manufactured with pressing technology (> 0.2 mm) or tape technology (0.05 to 0.2 mm). The all-ceramic transducers require high control voltages of 250 up to 1000 V. DuraAct Power patch transducers are based on a multilayer piezo element, whereby the operating voltage is only -20 to 120 V.


Functional Principle

The piezoceramic plates that are used in all-ceramic DuraAct patch transducers are similar structurally to a capacitor. The ceramic serves as a dielectric between the metallic surfaces of the ceramic, which constitute the electrodes. When an electric potential is applied, a field with lines running through the ceramic perpendicular to the plates is created. This causes a staggered contraction of the ceramic at 90° to the field lines so that the actuator constricts evenly along the plane. This behavior is called the piezoelectric transverse or d31 effect.

For the Power DuraAct patch transducer, the multilayer piezo element uses the longitudinal or d33 effect. The displacement occurs parallel to electrical field E and the polarization direction of the piezo actuator. The d33 piezoelectric charge coefficients for the longitudinal displacement are considerably higher than the transversal displacement and the possible displacements are greater than for the all-ceramic transducers.

The electrical field strength determines the displacement of the ceramic. This allows easy control of the modules. This deformation is transmitted effectively to the structural elements by an uncomplicated bonding. The force is transmitted over a surface by thrust and not at discrete points as is the case with conventional actuators. Massive force transfer points are therefore unnecessary. Conversely, structural deformations are converted to an electrical charge by the transducer making it possible to use the element as a sensor or power generator.

The reaction to a change in the electrical field or a deformation is extremely fast. Not only can oscillation be generated into the kilohertz range but also be measured. Depending on the active piezo element used and its dimension, other values with respect to control voltage and contraction result for various actuators. The relationship between deformation and control voltage is not linear.


Technology

DuraAct patch 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 transducer design features a piezoceramic foil with metalized surfaces for electrical contact.

The thickness of standard foils used is typically 100 to 500 μm, 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 robustAn alternative design features multiple layer piezo ceramics, enhancing force generation for the same operating voltage. 

DuraAct patch 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.

A graph of applied force versus actuator displacement is called the actuator characteristic curve. 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. 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. 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 the image.

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 to the substrate thickness and elasticity.

The 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.

 Pm = C · f · Uh2

f: Frequency

Uh: Voltage swing

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

Pmax = Pm ・ π