Design of a prototype position actuator for the primary mirror segments of the European Extremely Large Telescope
2010
European Extremely Large Telescope (E-ELT) based in 984 primary mirror segments achieving required
optical performance; they must position relatively to adjacent segments with relative nanometer accuracy.
CESA designed M1 Position Actuators (PACT) to comply with demanding performance requirements of EELT.
Three PACT are located under each segment controlling three out of the plane degrees of freedom (tip, tilt,
piston). To achieve a high linear accuracy in long operational displacements, PACT uses two stages in series.
First stage based on Voice Coil Actuator (VCA) to achieve high accuracies in very short travel ranges, while
second stage based on Brushless DC Motor (BLDC) provides large stroke ranges and allows positioning the
first stage closer to the demanded position.
A BLDC motor is used achieving a continuous smoothly movement compared to sudden jumps of a stepper.
A gear box attached to the motor allows a high reduction of power consumption and provides a great
challenge for sizing. PACT space envelope was reduced by means of two flat springs fixed to VCA. Its main
characteristic is a low linear axial stiffness.
To achieve best performance for PACT, sensors have been included in both stages. A rotary encoder is
included in BLDC stage to close position/velocity control loop. An incremental optical encoder measures
PACT travel range with relative nanometer accuracy and used to close the position loop of the whole actuator
movement. For this purpose, four different optical sensors with different gratings will be evaluated.
Control strategy show different internal closed loops that work together to achieve required performance.
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