Piezo Linear Actuator Design Priciples

There are two principaldesigns for piezo drives:

• direct actuation with CMA, multi-layer stack actuator. The stack directly position the load
• amplified piezoelectric actuators. A stack is supplemented with special gear designs or structural amplification mechanism of displacement e.g. when bender designs are applied (smart structure)

Direct Actuation is straightforward and self-explaining. The page focus on the versatile designs of stroke amplification.

Stroke of bender actuators are relatively high and can easily reach 1mm. Acoustic applications of benders are widespread. Telephone capsules for speaker and microphone functions, ultrasonic cleaner, valves driver, pump driver are known.

Piezo drives based on bending structures deploy stress-induced deformation and generate displacement like bimetal strips. Bender consists of several layers e.g. a piezo (P) and a metal sheet (M). The layers are glued together. Common bender variants are P-M, P-P, and P⁺-M-P^-. The superscript +/- indicates opposite phases of displacement (expansion, contraction). The piezo layer can also be a thin multilayer actuator (low profile multilayer stack actuator). The common piezo bender deploys d₃₁ contraction effect whereas the d₃₃ effect is effective in multilayer benders for higher performance. When the piezo layer is activated, the adjacent layers of the bender are subject to different stress levels. A torque is generated which deforms the bender. 

 Smart Materials and Multifunctional Structures represent research streams which focus on the development of advanced multi-functional structures. Multifunctional structures have in addition to their load carrying function additional functions of actuation, sensing and energy harvesting. Active materials such as piezo and shape memory alloys were prepared and integrated into fibre reinforced plastics. Due to the commendable stress and strain of the smart material constituent od the smart structure, the shape is commendable by changing the command. The stress which is generated by the smart material constituend transfer stress in the smart structure body and it will deform WOW!  For actuation in smart structures strain induced structural actuation mechanism was developed. Solid-state actuator components such as piezo plates, low profile stacks, or shape memory alloy wires were implemented into composite structures together with electrical wires and discrete sensors. The structures are able to generate e.g. twist or bend and significantly deform a smart material plate. Applications of those active structures are of importance in the domain of aeronautic applications such as active vibration damping and aerodynamic control. The latter application is highly attractive for gapless and  smooth aerodynamic elements. The motivation for these novel technology is about to alter the conventional way of engineering with its segregation of structures and systems. Also embedded sensors and systems were developed for health monitoring, self-diagnosis of the structure and some of theses approaches were already implemented in civil buildings and tested in aircraft applications.

The actuation approaches are manifold and distributed as well as concentrated piezoelectric actuators were integrated in structural configurations. Just to name a few, bending as well as strain-twist-coupling effects were elaborated in large projects.

Piezotechnik stack actuators provide active strain of approximately 1000 to 1100 micro-strain (0.1 to 0.11%) when driven with unipolar voltage and up to 1400 micro-strain when they are driven in bi-polar mode. The standard range of displacement is 10 – 100 micrometer. This is perfect for precision positioning and many other actuation applications like valve control, vibration management. When larger displacements are required a gear may be used to meet application needs. The quality and effectiveness of a gear for piezo is characterized by the preservation of the elastic energy. The elastic energy is ½ free stroke x blocking force. Deformations of the load transmission elements cause losses of the elastic energy and the potential to perform work at a load is reduced. Thus, gears for piezo stack actuators are special designs. Flexural hinges are deployed to minimize play and wear.

The next figure shows the principle mechanism of an amplified actuator.

Deformation of the lever under load is an obvious disadvantage of a lever-type design approaches. Any deformation in the mechanism reduces the work potential and the output force of amplified actuators. An elaborated model is presented in the next figure. The device is installed in the outer area of a rotor blades of an helicopter withstand very high mechanical loads and g-forces. The mechanism is numerically optimized for lowest weight and has very high stiffness of the levers as well as flexibility. This piezo stack actuators provide very strong forces and allow fast and precise control of aerodynamic flaps.

Source: ICAS 2008, PIEZO ACTIVE VIBRATION AND NOISE CONTROL IN HELICOPTERS