Actuator grid design for an adaptive deformable mirror

In a collaboration of the Technische Universiteit Eindhoven, Delft University of Technology and TNO Science and Industry a new adaptive deformable mirror (DM) is being developed. The deformable mirror will be used in large telescope systems to correct both the temporal and spatial high frequent distortions in the optical wavefront caused by atmospheric turbulence.The DM consists of a thin continuous membrane which acts as the correcting element. A grid of low voltage electro-magnetical push-pull actuators, - located in an actuator plate -, impose out-of-plane displacements in the mirror’s membrane. In the design variable reluctance actuators are used. These consist of a closed magnetic circuit in which a strong permanent magnet provides a static magnetic force on a ferromagnetic core which is suspended in a membrane. By applying a current through the coil which is situated around the magnet, this force is influenced, providing movement of the core. This movement is transferred via a rod imposing the out-of-plane displacements in the reflective deformable membrane. In the actuator design a match is made between the negative stiffness of the magnet and the positive stiffness of the membrane suspension. If the locality of the influence functions, mirror modes, force and power dissipation are taken into account, a resonance frequency of 1500 Hz and an overall stiffness of 1000 N/m for the actuators is needed. The dynamic response of the first actuators is tested in a dedicated setup. It shows an eigen frequency of 950 Hz. This is due to a lower magnetic force than expected. With a Helmholtz coil test setup the 2nd quadrant of the B-H curve is reconstructed by stacking of the magnets and using the demagnetization factor. It is shown that the values for Hc and Br of the magnets are indeed lower than the values used for the initial design. New actuators, with increased magnet thickness, are designed, fabricated and tested.Transfer functions are made with different dc offsets of the actuator. From these transfer functions, characteristics as resonance frequencies, force, stiffness, motor constant, current and dissipation and efficiency all as function of the stroke of the actuator have been adopted. These experimental results are compared with the theoretic predictions. The results are in good agreement.Grids with 61 actuators are made including the dedicated drive electronics. The module is the standard building block from which large arrays are assembled.