Low Profile Piezo Actuators Based on Multilayer Technology

摘要

Low profile actuators are a basic technology for smart structures. Bonded on surfaces or embedded in composite structures they act as actuators and sensors to control the structural behaviour. The simplest types are based on thin piezoceramic plates (typical thickness 200μm) provided with surface electrodes to operate in the lateral d31-mode. This type of actuator is able to generate strains of 500μm/m. To achieve higher deformations it is necessary to use the d33 -effect. The difficulty is to generate the necessary in-plane electrical field. A common solution is the use of interdigitated electrodes consisting of two comb like electrodes with opposite polarity that are placed on the surface of the piezoceramic material. Several types of d33-actuators have been realised using interdigitated electrodes. Especially actuators incorporating piezoelectric fibers or ribbons showed good performance. Known as Active Fiber Composites (AFC's) or Macro Fiber Composites (MFC's) these kinds of actuators can produce strains of 1600um/m. The drawback of interdigitated surface electrodes is a very high driving voltage of up to 1500V. In addition to that the electrical field is very inhomogeneous with high gradients causing mechanical stresses in the ceramic material that can lead to cracks. A promising concept to overcome these drawbacks is presented. It is based on the use of multilayer technology for low profile actuators. Standard multilayer configurations are d33-stacks or d31-bender. Within these actuators the electrodes are incorporated in the piezoelectric material during the sintering process as very thin layers with little impact on the actuator stiffness. This allows a significant reduction of the electrode distance and therefore also a reduction of the driving voltage. To utilize the multilayer technology for low profile actuators standard multilayer stacks are diced into thin plates (typical dimensions 5×30×0.3mm~3). In this configuration the electrodes are not only on the surface of the piezoelectric material but cover the whole cross section. This guarantees a homogenous electrical field in the actuation direction to exploit the full potential of the d33 effect. In a second step these plates are embedded into a polymer to build a piezocomposite. Without the mechanical stabilization of the surrounding polymer the handling of the fragile multilayer plate would be extremely difficult or nearly impossible. Especially a reliable electrical contacting of the collector electrodes is a key issue. Several prototypes have been build and achieved an active strain of 1200μm/m at a voltage of 200V. Using other materials an active strain of 1600μm/m is possible. With the large scale production of multilayer stacks in automotive industry these actuators become affordable and offer an interesting and high potential alternative to conventional actuation concepts for smart structures.