Piezo actuators are commonly used within Fast Steering Mirrors (FSM) for active stabilization, pointing and tracking functions. Such compact mechanisms are requested for Free-Space Optics and Deep Space Optical Communication since they are embedded and offer fast (up to 1kHz) and precise (μRad) tip tilt motion (up to +/-2°). The use of large amplified actuator within mirror telescope is new and become relevant since it displays enough power, reliability and do not fall apart when a failure occurs: steady state design with high stiffness 64N/μm. CTEC has developed and qualifed the largest amplified piezo actuator ever integrated in a telescope tip-tilt mirror of more than 2 meters diameter.
The design of the piezo actuator is based on the patented APA® structure from CTEC: An Elliptical shell magnifies the active material stroke up to 700μm and preload the piezo element for allowing large dynamic application. A parametric optimization study is realized to define the most efficient actuator reaching up to 44kN of Blocked force. Since the humidity limits the piezo component lifetime, the design includes innovating encapsulation method to guaranty 35 years of operation in large telescope conditions. The qualification of these large to justify 30 million cycles at full stroke and the full thermal evaluation.
The high preload applied to the piezo-electric material ensures a good behavior in dynamic mode, while preventing micro-slippage of the preload line. The actuator combines a very high blocked force in compact dimensions, while its high stiffness also allows dynamic applications. For optical purposes, the thermal expansion and dissipation as possible. The system only dissipates 0.7W during actuation at 0.1Hz. This leads to less than 1K elevation in steady state at the actuator interface. A detailed analysis has been performed to ensure a lifetime higher than 30 years, on both aspect: first ensuring an encapsulation to protect the piezo-electric material from humidity, second ensuring significant margins of flexible part for fatigue. A stress and thermal analysis have been performed to guarantee a proper behavior from -5°C to +45°C in operational mode and from -15°C to +60°C non-operational. The thermal expansion has been compensated with thermal washer to reach 6um/K from design. The actuator is assembled with several position and force sensors, to verify the preload and dimensions. The coordinate measurement verifies ~10μm tolerances in flatness and ~20μm in parallelism between top and bottom interfaced.
An intensive qualification test campaign has been performed. In static, the stiffness has been measured in small displacement and large displacement with up to 10kN. The dynamic of the actuator has also been studied, in free-free conditions, block free conditions, but also with 200kg attached on both sides. Thermal cycling has been performed for verifying the stroke in operational temperature range. The self-heating of the actuator has been measured to 1K in continuous use at 0.1Hz.The most demanding test has been the accelerated lifetime study, for which the actuator was realizing its operational stroke 550μm in free-free conditions with two masses of 200kg. Helium tests and full stroke health tests were realized periodically to up to 22 million cycles.
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