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Chapter 5 Analysis,

Chapter 5 Analysis, Calculations and Experiments 5.1 Thermal Analysis It is a fact, that a thermal analysis must be performed for every component once reaching the outer space due to the hash conditions and, of course, the missing air. Thus, no convection occurs which normally supports the heat dissipation mainly of electronic components. Two VCA concepts were presented in section 4.3.2 named as moving coil and moving magnet. One of theses concepts shall eventually be selected with help of a thermal analysis to proceed the MSTS design. Figure 5.1 shows the thermal networks for the two concepts consisting of six nodes, each coupled by heat conduction and thermal radiation respectively. The applied physical parameters of the nodes are listed in table 5.1. They are defined in all conscience at this project stage. The ESATAN software was applied for the calculations. The node Housing (No. 6) shown in figure 5.1 was set to a constant temperature of 45 ◦ C acting as a boundary condition. Fixing the mangnet does obviously not allow an efficient heat dissipation of the moving coil by conduction, but just by radiation. This was taken into account for the definition of the thermal networks. As variables, the temperature of the electronic board, whereof the coil will be supplied, and the length of its power leads were defined for the moving coil concept. Analogous, the glue thickness between the fixed coil and its mounting structure was altered for the calculations of the moving magnet concept. 30

6 EL board 5 Housing 3 Fixed part 1 Coil 5 – ANALYSIS, CALCULATIONS AND EXPERIMENTS 4 Moving part 2 Magnet 5 Housing 3 Fixed part 2 Magnet 4 Moving part 1 Coil 6 EL board Figure 5.1: Thermal networks for the moving magnet concept (left) and moving coil concept (right). Connecting lines indicate conductive couplings and thunder lines indicate radiative couplings between the thermal nodes. The electronics board is abbreviated with EL board. Figure 5.2 shows the calculated coil temperatures for both VCA concepts for a maxi- mum allowed average power consumption of 0.6 W (tab. 2.1) and a maximum housing temperature 1 of 45 ◦ C. On the abscissa, the temperature of the electronics board is plotted as undependent variable. The glue thickness is fixed to 0.1 mm and the lenght of the power leads amounts to 40 mm. With the moving magnet concept, the coil does apparently not heat up to temperatures of > 50 ◦ C, whereas the coil reaches temperatures of > 110 ◦ C with the moving coil concept. A relatively big gluing area between the fixed coil and the mounting structure allows a high conductive heat dissipation. This can furthermore be improved when reducing the glue thickness. With this analysis, the design progress has definitely turned in direction of the moving magnet concept for the MSTS VCA, what furthermore supports the postulation about the power lead breakage discussed in section 4.3.2. Now the mass of the magnet must be defined as small as possible to achieve the required shutter blade’s open/close time. 1 The preliminary thermal calculations of the whole MERTIS instrument show a maximum temperature for the MSOP housing of 45 ◦ C at Mercury’s subsolar point. 31