Optimizing the remote sensing instrument of Taiwan’s FORMOSAT-8 satellite

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Figure 1: Appearance of the optical RSI payload on FORMOSAT-5 satellite (1). Source: FEA-Opt

Taiwan began designing and developing its first satellite in 1994 in collaboration with a U.S. space systems contractor. FORMOSAT-1 was launched in January 1999 and provided service until its retirement in 2004. FORMOSAT-2 was launched later in 2004, followed by FORMOSAT-3 in 2006. In 2010, FORMOSAT-5 passed design verification and now awaits deployment. FORMOSAT-5 will carry the first optical remote sensing instrument payload completely designed and developed by Taiwan itself.

Today FORMOSAT-8 is under development. This new satellite will inherit the experience from FORMOSAT-5 and will likewise carry an optical remote sensing instrument developed by Taiwan. Opto-mechanical systems integration will be led by the country’s Instrument Technology Research Center, with participation by the Opto-Mechanical Design Analysis (OMDA) Lab supervised by Professor Yi-Cheng Chen at Taiwan’s National Central University, which is responsible for weight reduction of the mirror supporting structure. To achieve the breakthrough improvements required in structural weight, the OMDA Lab is utilizing FEA-Opt Technology’s SmartDO design optimization, system integration and process automation software.

Optical remote sensing instrument payload—The optical remote sensing instrument (RSI), used to take images for ground surface observation, will be exposed to harsh environmental conditions including vacuum, large temperature changes, high launch acceleration and random vibration. Therefore both the optical quality and strength of the mechanical structure must be considered when designing the optical system of the RSI. Figures 1 and 2 show the appearance of the optical RSI payload on FORMOSAT-5.

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Figure 2: Appearance of the optical RSI payload on FORMOSAT-5 satellite (2). Source: FEA-Opt

Building up the design optimization model and process—Delivering high performance together with low weight requires optimizing the structure of the mirror. To accomplish this, the development team is utilizing SmartDO as its toolset and platform for design optimization. On the SmartDO platform, the team linked together CAD and finite element software to optimize the characteristic dimensions of the mirror structure, with the goal of achieving light weight, high stiffness and high-quality optical performance.

Figure 3 shows the design optimization process built on SmartDO, which includes seamless integration among SolidWorks, ANSYS and SmartDO. Under this architecture, the user defines design limitations and requirements such as allowable stress and dimensional constraints, then SmartDO drives CAD and CAE automatically to perform design optimization. SmartDO’s direct global search technology makes design sensitivity studies unnecessary, so the team can devote more resources to other tasks.

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Figure 3: Integrated design process with SmartDO, SolidWorks and ANSYS. Source: FEA-Opt

Analysis and design optimization of the mirror structure—The mirror structure in the FORMOSAT-8 RSI follows the design in the PLEIADES–KORSCH TELESCOPE space project by Thales SESO. The main supporting structure has a hexagonal frame with honeycomb substructure within (Figure 4). A buttress structure from the SOFIA satellite made by Reosc is used to reduce deformation on the edge of the mirror structure due to cantilever effect (Figure 5). Twelve design variables for the geometric parameters were defined; Figure 6 shows the geometry of the initial design.

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Figure 4: Mirror structure of KORSCH TELESCOPE. Source: FEA-Opt
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Figure 5: Buttress structure in the mirror structure of SOFIA satellite. Source: FEA-Opt
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Figure 6: Design variables of the new mirror structure (initial design). Source: FEA-Opt

The twelve design variables include the principal dimensions of the main structure, buttress structure and honeycomb substructure. The structures need to sustain the mirror’s weight under gravity, and to sustain the mirror under polishing pressure. Deformation in specified directions and locations must remain within allowable tolerances for different loading conditions. The design optimization formulation is defined as:

  • Design variables: Geometric dimensions in Figure 6, DS1~DS12
  • Objective function: To minimize the volume
  • Constraints: Deformation should remain less than allowable value under different loading, in different directions and locations

SmartDO significantly reduced weight of the RSI payload—After the model was defined by the user, SmartDO solved the problem in a fire-and-forget manner without tedious sampling or sensitivity studies. In all, SmartDO successfully reduced the weight from 9.8kg to 8.8kg. Verification by finite element analysis proved that all performance indices were at least equal to the original design. In this important breakthrough in development of the FORMOSAT-8 satellite, SmartDO enabled the team to verify the feasibility of weight reduction, and carry this experience forward for further advanced study.

Reference: Chia-Yen Chan, Bo-Kai Huang, Zhen-Ting You, Yi-Cheng Chen, Ting-Ming Huang, “Optimal Lightweight Design of a Primary Mirror on a Remote Sensing Instrument,” CSMMT2014, November 2014, Taichung, Taiwan.