The simplest actuators are direct piezoelectric actuators (DPA) which can produce displacements of the order of ten to one hundred microns and exhibit high stiffness. These actuators are robust when highly pre-stressed and may be used in active satellite structures without the need for a locking mechanism during launch.
To overcome the limited displacement of a DPA, an amplified piezoelectric actuator (APA) may be used. This device has an elliptic elastic amplifier which bends in response to changes in the shape of the piezoelectric actuator (Figure 1).
Figure 1. The APA50 series of
piezoelectric actuators.
An important feature of this amplifier is the absence of any hinges, making it robust and likely to survive a launch. The piezoelectric device is a European ceramic multilayer actuator (CMA), which produces a high strain of 1200 ppm for input of 200 V.
Technical description
A complete line of amplified piezoelectric actuators has been developed (Figure 2).
Figure 2. A complete line of
piezoelectric actuators.
These can provide a full range of displacements (from ten to five hundred microns) and forces (from fifty to eight hundred newtons). These actuators have been designed using the finite element method, which can predict the performance of the actuator and also its electrical capacitance and the stress distribution.
A piezoelectric actuator is capacitive device, whose capacitance is often very large, as much as 10 microfarads. Such a device presents a difficult load to its drive electronics, since a significant charge transfer rate is needed to achieve a fast response. In addition the actuator will produce electrical energy when submitted to a mechanical load. Linear amplifiers are not well suited as drivers, and switched power amplifiers are more appropriate to this task. They also have a higher efficiency than linear amplifiers under reactive loading in dynamic applications. For these reasons, a small switched amplifier SA75 has been developed for positioning applications. A recent development of this technique combines piezoelectric and frictional forces to make a piezomotor that may replace conventional stepping motors in the future.
Applications and commercialisation
The piezoelectric actuators and their drive electronics just described are commercially available from Cedrat Recherche. They can fulfil a wide range of requirements in spacecraft instrumentation:
