This article is part of a four-part series that will discuss the challenge of smartphone lens modules, from the conception and design to the manufacturing and analysis of structural deformation. This article is part four. It covers the simulation of explicit  dynamics for the camera lens and to that effect the impact on the optical performance. Ansys Mechanical and LS-DYNA are used to simulate the explicit dynamics during a sequence of impact and bouncing of the phone with camera on the floor. LS-DYNA solves the drop physics, which is then imported into Ansys Zemax OpticStudio Enterprise via the STAR Tools where the impact on the optical performance can be studied.

Designing Cell phone Camera Lenses Part 1: Optics
Designing Cell phone Camera Lenses Part 2: Optomechanical Packaging
Designing Cell phone Camera Lenses Part 3: STOP analysis by using STAR module and ZOS-API

Authored By Flurin Herren

Downloads

• See downloadable attachment at the bottom of this article 

Introduction

Together with the Ansys tools from the previous parts in this article series, Ansys Zemax OpticStudio, Speos, Mechanical, and Workbench, Ansys LS-DYNA (LS-DYNA) can be used to extend the simulation workflow into explicit dynamics. LS-DYNA is used in a wide range of analyses. One of its core capabilities, is  explicit dynamics. Ansys LS-DYNA is useful for the analysis of problems involving contact, large deformation, nonlinear materials, transient response, and/or problems requiring explicit solutions.  

The LS-DYNA Workbench system(WB LS-DYNA)allows one to run an explicit dynamics analysis for a model using the LS-DYNA solver. While it allows one to preprocess, solve and postprocess in one environment, this workflow needs a combination of WB  LS-DYNA and LS Prep-Post for advanced postprocessing

Similarly to Part 3 - “Designing Cell phone Camera Lenses Part 3: STOP analysis by using STAR module and ZOS-API” of this Article Series, this part also uses Ansys Mechanical to generate FEA Datasets. However, while part 3 is focusing on the importing the FEA Data using the STAR tools and ZOS-API for automation, part 4 focuses on generating results for explicit dynamics and looking at the optical performance in Zemax. Both workflows require STAR tools in Ansys Zemax OpticStudio Enterprise to work with the FEA deformations.

Introduction Finite Element Analysis with explicit dynamics

The optomechanical system of the cell phone camera (see part 2 for optomechanical design) is loaded into Ansys Workbench and is imported into an LS-DYNA Analysis System. In order to make the crash simulation more realistic the camera system is placed inside of a larger body which has the size and shape of a common smartphone device.

The simulation contains the transient sequence of the camera system dropping on a flat surface. The flat surface, which could be the floor, is marked red on the image above and is set as a Fixed Support. Fixed Support is a boundary condition which prevents a selected geometry or mesh entity from moving or deforming.

It Is assumed that the object is dropped from rest (initial velocity = 0) and falls due to gravity alone. So, the velocity at the time of impact can be calculated using the following equation:

Where, v= impact velocity, g = acceleration due to gravity (9.8 m/s²) and h = height from which the camera system is dropped. It is assumed that the phone with camera system is dropped from a height of 1.5m, which is about the height of a typical person`s hand, the impact velocity would be

This leads to the following initial deformation of the whole phone:

And of the following deformation of the lenses itself:

• Please noted that the visual deformation has been scaled up for demonstration purposes.

To analyse the influence of this drop test on the optical performance, deformation datasets for the individual lenses are required. To extract the datasets, a Named Selection for each lens face is created. Once the simulation is solved in WB-LSDYNA, the input file and results are read in LS-PrePost.LS-PrePost is a dedicated prep-post tool for LS-DYNA .In LS-PrePost , a script is run to export the deformation of specific faces(defined in Named Selection) to the right format so they can be imported into Ansys Zemax OpticStudio via the STAR Tools.

Simulation involves two steps and deformation datasets are exported from both:

• Impact Analysis: This is from 0-0.1ms of the simulation time when the impact happens
• Post Impact Analysis: This is 1 sec after impact state when the vibrations are allowed to damp down to avoid any unnecessary noise in the deformations

Loading FEA Data into Ansys Zemax OpticStudio

After the FEA Data sets have been generated in Ansys Mechanical, they can now be loaded into OpticStudio. As elaborated in Part 1 of this article series, the nominal cell phone camera system has been designed and it`s performance optimized in OpticStudio. The design of the lens system itself is based on a patent and contains five main lenses with aspherical shapes:

In order to analyse and compare the performance of the cell phone camera under the three main states, Impact, Post-Impact and Nominal, the FEA Datasets are imported via the Multiphysics Data Loader in the STAR tab at the top of the OpticStudio main window.

For each surface which represents a physical surface of a lens or optical component, a “Surface_deformation” dataset is assigned. As the coordinate system has not changed since the nominal geometry was exported out of OpticStudio, the datasets are aligned for the specific surfaces are set for the global coordinate system. If this is not the case, the coordinate system can be changed to local, or a user-defined transformation can be applied. After the datasets are assigned to the surfaces, the datasets can be loaded and fitted by clicking “OK (Fit Multiphysics Data)”.

Analysis of optical performance during different states

After loading and fitting the multiphysics data, the performance of the different states can now be analysed and, more importantly, compared. As this is a cell phone camera system, there are some analysis tools which lend themselves to be used during the analyse of the performance. In this case the following analysis tools are used for analysis and comparison:

• Image Simulation - This feature simulates the formation of images by convolving a source bitmap file with an array of Point Spread Functions. The effects considered include diffraction, aberrations, distortion, relative illumination, image orientation, and polarization.
• Wavefront Map - Displays the wavefront error across the pupil.
• STAR System Viewer (Deformation) - Displays a system-wide view of the Surface Deformations and changes in optical properties due to the fitted multiphysics data.

Nominal State

As the lens system has been optimized for this state, the Image Simulation is of very good quality. The wavefront error is rotationally symmetric with a max error of 0.225 waves. There is no deformation displayed, because no multiphysics data is applied. This will server as the baseline and as the “ideal” state of the performance.

Impact State

When the datasets from the impact state are loaded, it is clearly visible that the performance of the camera system can be deemed as unusable. The deformations are so high that the results of the Image Simulation and the Wavefront Map can be declared as “stale data”. Interestingly, the deformation magnitude of the lens system which can be seen in the STAR System Viewer. The average deformation is around 0.33mm which for an optical system, is too much to perform and yield any results of significance.

A big advantage of the STAR Tools in OpticStudio is that you can decouple the effects of the rigid-body motion from the effects of surface deformation. This can be accomplished with a simple tick box in the Structural Data Summary and can be toggled on or off at any time. In the dynamic graph below starts from the full deformation data, first the RBM portion is disabled and then the deformation effects are ignored all together:

In the analysis results which are shown above the RBM is included. Below the same analyses are shown, but this time the RBMs are excluded. This enables you to observe the higher-order deformations, which are important during optical analysis. The STAR System Viewer now shows an average deformation magnitude of about 0.025mm, which leads to a wavefront error of about 40 waves, compared with the nominal performance, which gives a wavefront error of about a quarter wavelength, this still indicates severe optical aberrations. Such a large wavefront error leads to a highly degraded image quality which can be seen on the Image Simulation.

• Fore more information on the comparison of RBM and higher order deformations: STAR 22.3 Active RBM Feature

Post-Impact State

The results of the post-impact state are shown below.

Looking at the deformation vectors of the STAR System Viewer, it is still interesting that in the edge zone of some lenses there is still deformation magnitude of about 0.025mm. However, it is clearly visible that the magnitude in the last lens, which is the IR Filter, dropped significantly. This leads to a performance which is still notably worse that the nominal state but leads to a more usable result. The wavefront Map shows an error of about ± 15 waves, this is still far exceeding an acceptable limit for such an optical system. The Image Simulation shows this direct connection between the deformation of the lenses to the distortion and aberration which would occur in the camera system. The object is recognizable but very blurry.

Conclusion

This part four of this article series, has shown how to use Ansys LS-DYNA within Ansys Workbench to simulate the explicit dynamics of a drop test of the cell phone camera module. With Ansys Mechanical the deformation datasets of an impact and a post-impact state have been extracted and processed to be used in Ansys Zemax OpticStudio. In Ansys Zemax OpticStudio, the FEA Datasets can be loaded via the STAR Module and assigned to the optical system. This way the optical engineer can study and compare the performance of the optical system under the effect of the deformations of the impact and the post-impact state.

Next Steps

In this example, the optical performance during a drop test was analysed. However, another application area of the LS-DYNA-Mechanical-Zemax Workflow could also be the study of vibration or accumulative impact.