Piezo stack actuator is a linear motor based on electrically controllable deformation of a solid body. The piezoelectric effect is understood as the linear electromechanical interaction between the mechanical and the electrical state in certain crystalline. The piezoelectric effect is a reversible electro-mechanical mechanism:
The inverse piezoelectric is deployed for actuators and is used for static positioning as well as dynamic actuation up to the production of ultrasonic sound waves.
The displacement of a piezo actuator is in first instance proportional to the voltage input. Forces are much higher than that of any other electrical device of comparable size such as a linear voice coil motor. It is extremely important to notice, that the force capability is associated with fairly small stroke.
The generation of strain for actuation requires field strength in the order of 1.000 - 2.000 Volt per mm layer thickness. As generally low voltage is required, many thin piezo layers are stacked to create actuators. This result in a body of piezo material which is furnished with an electrode structure. A design for the electrode structure is the well-known pattern of a MLCC (multi layer ceramics capacitor). Electrodes are insulated against anti-pole electrodes inside the stack. The next figure indicates, that such insulation method result in inhomogeneous field distribution, which cause inhomogeneous field which is associated with mechanical stress concentration (see circle).
On key design aspect of Piezotechnik actuators is the homogeneity of the internal field and an adequately optimized electrode structure. The applied electrical field directly produce internal mechanical stress in the ceramics. For best performance and efficiency uniform deformation of the stack is most desirable. Also, inhomogeneous field distribution cause high tensile stress components in a piezoelectric stack, which is detrimental to the mechanical integrity of ceramics body. Thus, homogenous field distribution is of fundamental importance for performance and life of a piezo stack actuator. Piezotechnik stacks have full stack-through electrodes which cover the whole area of the active layers. The stacks have external insulation on the sides to insulate the connector which links the electrodes of same polarity. This geometry is the only solution for an ideal homogeneous field distribution.
The advanced solution is presented in the next figure. The electrodes are insulation on the faces of the stack. The insulation alternates for each pole and the electrodes of common polarity are connected by a metallic strip. This result in a perfect internal electric field pattern.
The benchmark solution for piezoelectric actuators is the co-fired Multilayer Actuator (CMA) technology. In the CMA production process the whole actuator stack structure including internal electrodes and piezo ceramics material is produced in one step. Electrode and ceramics raw materials are formed to a block and sintered at high temperatures to a monolithic block. This process is named co-firing. This design he best design of a piezo plate with regard to deformation and associated mechanical stress distribution is a perfect electrostatic capacitor, where two parallel electrode plates fully cover the piezo material and generate a homogenous electric field.
The advanced piezo stack comprises a pile of those piezo layers to increase the displacement performance. The electrodes cover all the layers and are open at side faces of the stack. The layers are contacted with metallic stripes at the side face where antipole electrodes are insulated.