Principles of Piezo Drive - Piezo Stack Actuator Design

 

Overview on piezo drive designs

 

There are two principal classes of piezoelectric stack actuator designs for piezo drives:

  • direct actuation with CMA, multi-layer stack atuators with comprise a large number of stacked thin layers (less than 100 µm),
  •  amplified piezoelectric actuators with special gear designs or structural amplification of displacement e.g. with bender designs (smart structure).

Piezo drives built as bending actuator structures

 

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.

 

 

Beyond piezo drives - Smart Materials and Multifunctional Structures.

 

Smart Materials and Multifunctional Structures represent research streams which focus on the development of advanced multi-functional structures. Multifunctional structures have in addition to a load carrying function additional features of actuation, sensing and energy harvesting. Active materials such as piezo and shape memory alloys were prepared and integrated into fibre reinforced plastics. 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 sensors. The structures are able to generate twist or bend and significantly deform. Applications of those active structures are in the domain of aeronautic applications such as active vibration damping and aerodynamic control purposes. The latter application is highly attractive for gapless smooth aerodynamic elements. The motivation for these novel technologies is about to alter the conventional way of engineering with 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.

 

Piezo drives designed as amplified stack actuators

 

Piezotechnik stack actuators provide active strain of 1000 micro-strain (0.1%) and 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 a amplified actuator.

Principle design of amplified piezo stack actuator
Principle design of amplified piezo stack 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.

Amplified piezo actuator, high stroke and force for vibration and noise reduction
Amplified piezo actuator for helicopter rotor blade, 800N, 1.5mm fully functional under 800 g centrifugal load

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