How to Preload Piezoactuators?
In most applications, piezoactuators are need to be preloaded for safe operation and optimum performance. That means they are under a compressive stress even when they are not operational. This is due to prevent tensile forces acting upon the ceramic. When they are not preloaded, especially in high frequency applications (such as a piezo shaker or an FSM carrying a large mirror), tensile forces could be enough to break the actuator. Temperature differences can also cause shrinkage on ceramic that may cause tensile stresses.
Like other ceramics, piezoactuators are very weak according to tensile loads. They can withstand much more in compression. Therefore, maintaining a compressive force on the actuator during operation is beneficial. Manufacturers of multilayer piezo actuators recommend a preloading force between 10-40% of the blocking force of the actuator. Exact force should be determined by calculating the forces on the actuator, stress limits of the preloading structure and desired stiffness.
Without preload, Piezo may be subjected to tensile forces (positive). Preloading maintains a compressive load on the actuator to ensure safe operation
Maintaining a stable preloading force on the piezoactuator is not trivial. Below, you can see some of the methods that can be implemented. Many other alternatives are possible but mostly a spring-like structure is needed such as;
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A coiled spring is a good choice since their spring constant is low compared to other alternatives thus allows precise adjustment of the preload force. But it can be bulky for small devices.
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Conical spring washers are good for their small size. They can be stacked in series to reduce the spring constant or series to increase their load capacity but they may not give repeatable results because of manufacturing tolerances.
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Using sheet metal springs also good for limited space, they can be precisely manufactured with laser cutting in small sheets and stacked to increase their load capacity.
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Using a compliant structure is a good choice when other alternatives are too bulky or not precise enough. It can be manufactured with EDM that gives good precision.
In the image, you can see a lever like design that utilizes a S shaped structure to be tensioned to preload the piezo. A conical spring washer is also useful thanks to their high load capacity and small size. They are stacked series to reduce their spring constant. A compliant cage can be used too, which is tensioned before assembly and released when the piezoactuator is placed in the cage.
Left: A lever structure with S shaped spring to be tensioned with a screw to apply preload.
Middle: An enclosure with a series of conical spring washers pushing on the piezo
Right: A compliant cage that is pretensioned before assembly, then released to apply compression on the piezo.
Stresses on preloading structures can be calculated with FEA
A thorough analysis needs to be done to ensure enough preload is applied on the actuators. If the spring constant of the structure is high, small variations of dimensions could lead to significant variations on the preload force. Keeping stress values low is crucial too, especially in high frequency applications where cyclic stresses can lead to fatigue failure. Finite element analysis is practical since the shape of structures are mostly too complex to analyze with hand calculations.
Preloading with a stiff spring would reduce the displacement output of the actuator. Your actuator and the preloading structure wold act like a parallel springs. If your application needs large displacements, make sure your preloading spring is compliant enough. Moreover, piezo contraction would reduce the effective preload. Try to make the stiffness of the preloading spring is lower than the 10% of the piezoactuator's stiffness.
In most applications, you will need preloaded piezo actuators. Our Amplified Piezo Actuators are designed and assembled so that enough preload is applied on the ceramics. If your application needs a different solution, you can contact us to explore new design ideas.