| | | Piezo Actuators with Integrated Lever Motion Amplifiers Piezo actuators or positioning stages can be designed in such a way that a lever motion amplifier is integrated into the system. To maintain sub-nanometer resolution with the increased travel range, the lever system must be extremely stiff, backlash- and friction-free, which means ball or roller bearings cannot be used. Flexures are ideally suited as linkage elements. Using flexures, it is also possible to design multi-axis positioning systems with excellent guidance characteristics (see p. 4-43). PI employs finite element analysis (FEA) computer simulation to optimize flexure nanopositioners for the best possible straightness and flatness (see Fig. 49 and Fig. 51). Piezo positioners with integrated motion amplifiers have both advantages and disadvantages compared to standard piezo actuators: Advantages: ■ Longer travel ■ Compact size compared to stack actuators with equal displacement ■ Reduced capacitance (= reduced drive current) Disadvantages: ■ Reduced stiffness ■ Lower resonant frequency The following relations apply to (ideal) levers used to amplify motion of any primary drive system: | | where: | | rience in micromechanics and nanomechanisms. A balance between mass, stiffness and cost must be found, while maintaining zero-friction and zero-backlash conditions. Coupling the piezo stack to the lever system is crucial. The coupling must be very stiff in the pushing direction but should be soft in all other degrees of freedom to avoid damage to the ceramics. Even if the stiffness of each of the two interfaces is as high as that of the piezo stack alone, a 67 % loss of overall stiffness industrial still results. In many piezo-driven systems, the piezo stiffness is thus not the limiting factor in determining the stiffness of the mechanism as a whole. PI piezomechanics are optimized in this regard as a result of more than 30 years experience with micromechanics, nanopositioning and flexures. | | |
| | | ALSys = travel of the lever-amplified system [m] ksys = stiffness of the lever-amplified system [N/m] k0 = stiffness of the primary drive system (piezo stack and joints) [N/m] fres-sys = resonant frequency of the amplified system [Hz] fres-0 = resonant frequency of the primary drive system (piezo stack and joints) [Hz] Note: The above equations are based on an ideal lever design with infinite stiffness and zero mass. They also imply that no stiffness is lost at the coupling interface between the piezo stack and the lever. In real applications, the design of a good lever requires long expe- | | |