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Different actuator designs

There are different ways of using the piezoelectric effect for actuator purposes: with monolayer, multilayer, stacks, benders or amplified actuators. 

Definitions: axes, free stroke, blocking force

There are a few definitions that are important to get in place in order to get the full understanding of actuators, actuator design and how to select the right actuator: axes, free stroke and blocking force.

Axes

Similarly to most materials, piezoelectric ceramic is elastic, i.e. it follows Hooke’s law. In other words, strain is proportional to stress. In addition, the piezoelectric effect can be represented as an additional term in the relationship, in a first approach proportional to the electrical field. This can be visualized as a relationship on two axes. This ”bilinear” relationship is valid at the material level (strain, stress, field) whatever the orientation of the material and can be extended to all types of actuators (displacement, force, voltage). As a result, any piezoelectric actuator can be described as a spring with a characteristic that can be shifted through the application of a voltage. Or in a displacement vs. force graph.

Free stroke and blocking force

It is an industry standard to describe this behavior using the terms ”free stroke” and ”blocking force”. Free stroke is the vertical distance between the curves while blocking force is the horizontal distance between the curves. Note that in this model, the stiffness of the actuator is (blocking force)/(free stroke) and is supposed constant.This characteristic might be limited by a minimum/maximum force allowable on the actuator. However, these parameters are not necessarily related to the blocking force.

Further reading: Constitutive equations

Monolayer components

The most common configuration of a monolayer component is with the electrodes perpendicular to the poling direction. Here, the application of an electric field on the surface electrodes will lead to a contraction in directions "1" and "2" together with an expansion in direction "3". Monolayer components are available as rectangular plates, discs, rings, tubes. As actuators, they provide micrometer displacement but require high voltage for operation (kV range).

Multilayer actuators

Noliac’s plate and ring actuators (NAC20xx and NAC21xx) can be used either in direction ”3” or ”1” similar to monolayer components. The difference compared to monolayer components is that the active volume is divided into many thinner layers, allowing high electric field under lower voltage. Free displacement is up to 5 µm with blocking force in the range 200 to 10000 N depending on material and cross-section.

Stacked actuators

NAC20xx and NAC21xx actuators can be stacked in order to multiply the displacement. As a result, free displacement is proportional to the height of the stack. Blocking force is almost unchanged.

Shear actuators

In Noliac shear plate actuators, the electrical field is applied orthogonally to the poling direction. The resulting motion is a “tilting” motion leading to a translational movement in the plane of the electrodes. Shear plate actuators can be stacked in single-axis and multi-axis solutions.

Benders

Benders
Noliac’s bender plates and bender rings (CMBP and CMBR) exploit the contraction in direction ”1” and amplify it through a bending motion. As a result, large free displacements can be achieved (up to the millimeter range).

Amplified actuators

In order to obtain even larger displacement, actuators can be coupled to an amplifying mechanical structure. The trade-off is a lower stiffness and blocking force. Many schemes can be found (lever, flextensional, hydraulic etc.). Noliac’s own amplification mechanism is commonly called ”diamond frame” and is based on the differential expansion of two sets of multilayer stacks in a ”V” configuration.

Comparison of all actuator designs

The different actuator designs provide a wide range of performance, each with their preferred characteristics. Blocking force and free displacement performance can be plotted on a graph for the different actuator families, providing an overview of available products. Custom designs can of course expand the plotted areas.

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