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Tiny piezo-hydraulic actuator delivers 150N

20 October, 2014

Researchers at Siemens have developed a powerful small actuator that combines piezoelectrics with hydraulics to produce a 9cm-long device that can apply a force of more than 150N and movements of up to 2cm. They say that the piezo­hydraulic actuator could be used to operate valves and flaps, to drive aircraft ailerons, and in robotic, medical and cleanroom applications.

Siemens sees the device as an alternative to electromagnetic actuators which lose efficiency when they are very small. Also, once the new actuator has been set, it maintains its position, unlike conventional actuators, which can be affected by vibrations and other influences.

In the piezo-hydraulic actuator, a voltage causes tiny deflections in a piezoelectric crystal. These small movements are combined hydraulically to generate a movement of 2cm.

When a voltage is applied across a piezoelectric crystal, it expands in a particular direction. They are used, for example, to drive injection valves in combustion engines. Due to their high stiffness, they respond with almost no lag. By contrast, conventional hydraulic systems need a central pump to compress the hydraulic fluid in all of its pipes, resulting in a delay before they can produce movement. 

The new actuator achieves a high level of stiffness by using only 6ml of hydraulic fluid. The enclosed hydraulic system consists of three adjacent metallic bellows that can expand along an axis and are connected to each other by non-return valves. When the piezoelectric crystal is excited, it expands into the central chamber, where it creates pressure. This pressure opens the valve to the adjacent chamber, which has an actuator stem attached to its front. The inflowing fluid expands the bellows slightly, thus extending the actuator stem.

Siemens' tiny piezo-hydraulic actuator can deliver forces up to 150N

The piezoelectric crystal is driven by a high-frequency sawtooth voltage and the rapid succession of small expansions combine to move the actuator stem smoothly. If a reverse voltage waveform is applied, the direction of pumping and motion are reversed.

The actuator’s metallic housing encloses all of the hydraulic fluid inside the system, so the device only has to be supplied with electricity, not fluid. It is protected against external influences such as dust, humidity, and chemicals.

The system is a development of an earlier piezohydraulic actuator that researchers at Siemens' global corporate technology research department created to control the valves of large combustion engines, such as gas turbines.

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