Authored By Hui Chen

Introduction

Structural, Thermal, Optical Performance (STOP) analysis enables optical engineers to include performance impact from mechanical deformation and thermo-optic effect in their design evaluation. This meticulous analysis helps designers identify critical issues on a system-level so they can make design correction and deliver better products to market sooner. OpticStudio features Structural, Thermal, Analysis & Results (STAR) tools and analyses to streamline the STOP analysis workflow. Since its release in 2021 STAR has gained increasing attention in the industry and has become the go-to tool for STOP analysis for many of our OpticStudio users.
Our STAR team continues to develop new features and capabilities to make STAR more powerful and user friendly. In the latest Ansys Zemax OpticStudio release, we have added a new STAR FEA Symmetry tool. In this article we will discuss how to use this Symmetry tool.

Structural Symmetry

A full-scale structural analysis with refined meshing can take very long to run, depending on the complexity of the structure, the number of degrees-of-freedom (DOF), and the mechanical and thermal loads applied. FEA engineers are constantly looking for ways to reduce the run time and utilizing symmetry is among the top considerations. Taking advantages of the structural symmetry can help reduce the size of the task therefore shortens the computation time significantly. By properly applying symmetry conditions, mechanical engineers can model a portion of the system and get useful insights as if the entire system was modeled. As a rule of thumb, the time that a simulation takes to complete is reduced by n^2, where n is the fraction of the full system. For example, using symmetry to model just a quarter of the entire system can enable you to complete a simulation 16 times faster. This can cut run times from days to hours during the design cycle.
To load simulated datasets into OpticStudio, STAR loading tool requires the structural and thermal data to cover the entire surface or volume of the optic in order for the ray tracing algorithm to work properly. Missing structural or thermal data for part of the surface or volume may lead to ray tracing error and cause rays to terminate prematurely. This means even if the structural model possesses certain symmetry, the FEA engineer still needs to model the entire geometry and will not be able to take advantage of simulation time reduction due to symmetry. Instead of simulating just a quarter or half of the system, they will have to run simulation for the entire system, which could be extremely time consuming. In consideration of this workflow, we have developed this FEA Symmetry tool. Using this tool, customers can turn partial structural or thermal data into a complete dataset using mirror symmetry or rotational symmetry. This tool enables FEA engineers to utilize symmetry as much as possible to greatly reduce the FEA simulation run time and thereby the overall STOP analysis time.

FEA Data Viewer

After receiving structural and thermal data from the FEA engineer, optical engineers can use the FEA Data Viewer to access the dataset to check and confirm if the dataset has been properly formatted and is readable by the STAR tools.
Under the STAR tab, you’ll find the FEA Data Viewer tool at the very left. Click it to open and in the File Explorer window that appears, navigate to the folder that stores the FEA data and select the files you want to examine. The tool lists the number of mesh points contained in the data file. It displays the Maximum, Minimum, and Median of the node positions in X, Y, and Z directions as well as the deformation Maximum, Minimum, and Median in X, Y, and Z directions. Reviewing these numbers can give you a rough idea if the data makes sense. The 3D Viewer on the right displays the node positions with a color scale based on the deformation magnitude (default setting for structural deformation files; for temperature profiles the scale is °C) to help you visualize the shape and size of the FEA data. In this example below we see the structural data only covers a quarter of the entire surface. This indicates the FEA engineer has utilized symmetry and only performed FEA analysis for a quarter of the optic. A partial structural dataset cannot be used directly in OpticStudio. However, with the help of the STAR Symmetry Tool, we can mirror or rotate the quarter dataset to generate a dataset that completely covers the optical surface before loading it into OpticStudio with the STAR Load FEA Data tool.

Using the Symmetry tool

The Symmetry tool is located under the STAR tab, the second button to the left. Clicking this button opens up a window as shown below. Press the “Open FEA file(s)” button to navigate to the FEA data folder and select the file(s) you want to work with. In this article we use the Inverse_telephoto_lens.zos file as an example. After loading the selected datafile you will see data points displayed in False color in the graphics area. There are two symmetry options you can utilize: Mirror and Rotate. These options will be described in the following sections.

Mirror Symmetry

With the Mirror option, you can mirror the FEA dataset across one or more of the three mirroring planes, XZ, YZ and XY. For example, starting with the quarter data below, you can first choose to Mirror about XZ plane to create data covering the left half of the circle, and then Mirror about YZ plane to reach a complete circle of structural data. Once you have made your selection by clicking on the XZ and YZ checkboxes, you can click the “Preview Selected” button to see a preview in the graphics area.

Rotate Symmetry

In the above example with quarter symmetry, you can also use the Rotate option to complete the dataset. For the Rotate option, user can choose the rotation axis from one of the three main axes, X, Y, or Z. Selecting the Z axis of rotation in this case and then clicking Preview will show a dataset that covers the entire optical surface.

Special Cases of 2D Assumptions

In some instances, and typically early in the design process, the FEA engineer can reduce the complexity of the simulation from 3D to 2D. This type of assumption drastically reduces the simulation time and enables teams to do more rapid design iterations using the insights gained from more frequent simulations. Please keep in mind that this approach may not be applicable in all cases and results should be used to direct the design and not for final analysis.
To demonstrate working with a 2D FEA dataset, the data below is just a single curved line of data points along the radial direction on a spherical surface. In this case the FEA engineer took advantage of a 360 deg symmetry during the FEA analysis. Using the Rotate option and setting the rotation axis as the Z axis, the tool will rotate the data around Z axis 360 degrees to form a dataset covering the entire circular surface region. Click Preview Selected to see that the tool has created an FEA dataset that completely covers the optical surface starting with just a single line of the FEA mesh nodes.

Thermal data profiles can also be used in 2D analysis. Below the temperature data provided by the FEA engineer is just a single slice of temperature points on the XZ plane. Revolving this slice around Z axis using the Rotate option will form a complete temperature point cloud that occupies the entire 3D lens volume.

Working with Multiple Partial FEA Data Files Simultaneously

Now imagine you have received many sets of such partial structural and thermal datasets from your FEA engineer. In order to use them in STAR for ray tracing, you will need to perform mirroring or rotating actions for every one of such dataset which becomes tedious and time consuming. The Symmetry tool supports the option to load multiple partial data files so you can apply the same action to all of them at once. As long as these files possess the same symmetry assumption and require mirror or rotation about the same symmetry plane or axis. After selecting the files and applying proper Mirror or Rotation motion, you can click the Preview All button to see how all of the loaded FEA datasets will appear after the mirror or rotate function is applied. The Clear Preview button removes all generated dataset from the graphics area to reset it in case you need to start over.

Saving New Files

After you have generated preview for the partial FEA dataset(s) and are satisfied with the preview of the complete dataset, you can then click the “Save New Files” button to save these generated datasets. These newly saved datasets can then be used to import into your system for STAR ray tracing. These new data files created by Symmetry tool share the same file name as the original data file, with a suffix of “_mirrored” or “_rotated” to help you differentiate the new file from the original file.

Load Complete FEA Datasets for STOP Analysis

After you finish creating these new data files, you are now ready to load them into your optical model using the Load FEA Data tool, just like you would do with normal FEA dataset. Once the structural and thermal datasets are in place, you can use the Structural Data Summary and Thermal Data Summary editor to view the list of all surfaces or volumes with FEA datasets applied to them, as well as the name of the data file applied. You can turn on and off datasets individually within the data summaries to include or remove it from ray tracing. This allows you to isolate and test the sensitivity of the structural or thermal impact of certain elements. In addition, you can also use the Performance Analysis tool to visualize the performance impact of each dataset. The tool enables you to see how each FEA dataset is contributing to the overall change in the specified performance metric. With this tool you can see which surface is contributing the most to the overall performance change. Furthermore, the analysis will differentiate between the performance change contributions from structural deformations and temperature data. The end result is the ability to not only see which surface(s) are contributing to the performance change but also the nature of that change.

Conclusion

In this article we have demonstrated how to use the new Symmetry tool in the latest Ansys Zemax OpticStudio release . When performing structural and thermal FEA analysis, symmetry can be considered during FEA simulations because it helps reduce the size of the model therefore shortens the simulation time significantly. OpticStudio cannot take in partial FEA dataset directly, but with the help of this Symmetry tool you can now transform partial FEA data, for example the half or quarter portion or even a single line of FEA datapoints, into a complete dataset that covers the entire lens surface and volume. This tool is a great addition to STAR capabilities. It empowers optical designers to work closely and seamlessly with FEA engineers to perform more efficient structural and thermal analysis and accelerates the whole STOP analysis workflow.