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6 – MERTIS SHORT TERM

6 – MERTIS SHORT TERM SHUTTER DEMONSTRATOR MODEL 6.2.1 MSTS FH Structure The maximum force generated from the Helmholtz VCA and the required first eigenfrequency determined by the switching mode (fig. 2.1) define the dimension parameters of the FH. They result from the FEM simulations to FV CA= 124 mN and f (1) eig = 110.4 Hz. The leaf width was defined to b = 4 mm and its length1 to l = 20 mm. For reaching an adequate spring constant c, the leaf thickness amounts to h = 80 µm. Thus, a necked down flexures design is necessary. With ξ = 0.5, the length of the flexible hinges results to lc = 5 mm for each leaf. With a thickness of 0.5 mm, the rigid part is around 250 times stiffer than the leafs. The parallel stage avoids canting and rotary movements of the magnet and increases the eigenfrequency due to the higher stiffness. However, this will be counteracted by the higher mass, what forced the implementation of three mass reducing cavities in the rigid parts. The infrared light beam exits the slit with a divergence of ≈ 30 ◦ and passes the MSOP window afterwards. So, the blade will be placed as close as possible to the slit for reducing the blade width. The force should be induced in the middle of the FH structure to avoid tensile and compressive stress in the leafs [4]. This would require an additional rigid bar and a changement of the VCA positioning. But FEM calculations showed maximum stress amplitudes in the leafs far below the endurance strenght limit when inducing the force at the bottom of the FH structure. Thus, the Helmholtz VCA is positioned centered to the half round magnet mounting area (fig. 6.1). Since the magnet must be placed axially centered between the coils, an additional support pin with the same diameter and a resulting length of 4.7 mm is required to connect the magnet with the FH structure. Aluminum was selected as material to keep down the moving mass. 6.2.2 Mounting Part The MSTS DM will be fixed at the MSOP structure with two M2 screws. The MSOP window and the milling slot for the grating adjustment screws (fig. 2.2) restrict the bore placement for the MSTS screws. Because the VCA must be placed towards the grating and, therefore, interfere with its screw, an additional knob at the MSOP is 1 The height of the MSTS obviously exceeds the dimension requirement with the defined leaf length. However, this was approved by the instrument prime. 48

6 – MERTIS SHORT TERM SHUTTER DEMONSTRATOR MODEL Coils Magnet Material MULTOGAN Material NdFeB dC 0.1 mm dM 2 mm hC 1.75 mm dL 3 mm lC 3.85 mm χ 0.15 mm d 1.3 mm BHmax 38 MGOe N 2 × 232 ζ 0.1 mm Table 6.2: Electromagnetic parameters of the MSTS DM design. necessary. The bores are designed as long holes for adjusting the shutter blade relative to the light beam. The mounting part supports the coils of the Helmholtz VCA. Due to the coils’ diameter of ≈ 6 mm, a slot must be milled in the MSOP to let insert the MSTS VCA support. 6.3 Electromagnetics As confirmed with test constructions, the Helmholtz VCA is capable to achieve an around 1.8 times higher FV CA than a VCA with an equivalent coil, without necessi- tating more installation space. Listed in table 6.2 are the magnet and coil parameters. Figure 6.2 shows the VCA cross section with the corresponding dimension parameters. A glue thickness of ζ= 0.1 mm was defined. The coils were hand-crafted and glued with epoxy after every winding layer to optain self-supporting air-core coils. The magnet was glued on the support pin, which itself was glued on the junction of the FH structure. A small vertical displacement of the magnet relative to the coils occurs at maximum deflection because of the parallel FH structure setup. However, the displacement for the required stroke of f = 1.5 mm amounts to approximately 56 µm. With a magnet clearance of χ= 0.15 mm, enough space between the magnet and the coils is foreseen, even for longer strokes possibly provoked by launch shakings. 6.4 Measurements Static and dynamic measurements were performed with the constructed MSTS DM to determine the mechanical and electromagnetic parameters, which are required for the 49

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