Combining the Power of ANSYS and the WEB - Automating Stabilizer Bar Analysis

By Joe Saxon & Chip Beaulieu
ArvinMeritor Light Vehicle Simulation & Analysis

t ArvinMeritor, Finite Element Analysis (FEA) is a routine part of developing a stabilizer bar design. The analysis itself is straightforward but time consuming. In the past, a delay typically occurred between the request for analysis, made by the design engineer, and the actual analysis performed by an FEA specialist. The analysis and documentation then required several hours of the specialist’s time. Productivity could obviously be improved if a program or macro could be created that drove the FEA software to produce the analysis and documentation, thereby freeing up the FEA specialist for more difficult projects. Creating this macro was clearly a role for the FEA specialist. Another factor to consider was the geographic diversity of the users. The design engineers’ offices are in Michigan and the United Kingdom, but the specialized FEA software and hardware is locally licensed to the analysis office in Tennessee. To make these specialized resources practically accessible to remote end-users required expert utilization of the corporate intranet. The complete solution required a combined Engineering/IT effort and resulted in a software tool that combines Active Server Page (ASP) technology with an ANSYS® APDL macro. Several difficulties had to be overcome to bring these technologies together, but the solutions opened the door for development of similar tools that efficiently distribute engineering solutions through web technologies familiar to the end user. This example epitomizes the goals and potential of ArvinMeritor’s Web Based Engineering (WBE) initiative.

THE FEA MACRO

OVERVIEW – A very significant portion of the efforts required to develop this tool were directed toward creating the FEA macro. The macro was required to start with a simple definition of the bar and its boundary conditions, then build the FEA model, run the analysis, interrogate the results, and write an HTML report. The macro capability in ANSYS proved ideal for this application. Figure 1 (see page 2) shows the user interface to the tool, which includes a diagram defining the input variables. Bar definition – Packaging constraints imposed by OEM’s generally define the path a stabilizer bar follows across the suspension. The real design issues for the stabar supplier are cross-section, material, and process. Typical bars have spherical joints at the ends that eliminate moment. Therefore, stresses at the ends are not an issue, and neither is geometry detail. Bar geometry is adequately defined by a cross-section and the centerline of the bar. The bar centerline is supplied to the tool as a small IGES file. Three cross-sectional choices are available in the current tool: 1) Solid Circular with constant diameter; 2) Tubular; 3) Solid Round with tapered diameter. Upon selection of a cross-sectional option, the subsequent menu changes to prompt the user for the appropriate information. Option 3 is the most complicated, requiring not only diameters, but also coordinates of points on the bar where those diameters apply. Between specified points, the diameter is linearly tapered.

FEA MESH – Each cross-sectional type is best modeled with a different element type. Solid Circular constant diameter is modeled with ANSYS Beam189 elements that are very computationally efficient. ANSYS Shell93 elements are used for tubular bars. Shells address some stress issues overlooked by beam solutions, but require greater solution times. Solid, tapered bars are best modled with brick and wedge shaped solid elements (ANSYS Solid45). Again, solid elements require greater solution time, but they provide post-processing advantages over tapered bar elements. The macro’s meshing logic ranges from very simple for the beams, to very complex for the tapered cross-sections. Convergence studies were performed to optimize the pre-determined mesh densities for accuracy and solution efficiency.

Figure 1 - Stabar Tool's User Interface

SOLUTION – In general, the solutions for each model are inherently linear. The only decision required from the user is a yes/no for additional post-processing that produces XY plots of solution values plotted along the bar’s length. Each model is solved on an SGI workstation featuring four parallel processors and one gigabyte of RAM. Remote access to a specialized and expensive machine of this type is one of the distinct advantages of this web-based tool. For the beam solution, turnaround is less than a minute. For shell and solid solutions turnaround is about four minutes, but increases to about ten minutes if the additional XY plotting is requested. Compared with traditional means, efficiency has been enhanced by roughly two orders of magnitude. The real benefit is even greater because no time is spent waiting in the FEA specialist’s input queue.

DOCUMENTATION – The macro also takes advantage of ANSYS’ ability to write almost any formatted statement to an outside file with the VWRITE command. The ANSYS macro actually writes the HTML report, completely documenting the input values and results with text and pictures. The text portion includes reaction force vectors, bar length and weight, and maximum stresses and strains. The graphics portion includes stress and strain fringe plots and XY graphs. The XY Graphs show results as a function of bar length. The macro programming required to produce these XY plots is quite simple for beam models, but quite complex for shell and solid models. This is not a pre-fab report made available through ANSYS, but a fully customized report designed by ArvinMeritor. Upon completion of the solution, this report is displayed in the end user’s browser window. As with anything seen in a browser window, the entire report can be printed as hard copy, or the user can right-click on the graphics and save them to a local drive.

ACTIVE SERVER PAGE TECHNOLOGY

OVERVIEW – The Web portion of this tool was built on Microsoft’s Internet Information Server (IIS) web server using their Active Server Pages (ASP) technology. ASP was selected because it uses the Visual Basic programming language, which is easy to learn and quite flexible.

INPUT DATA – Data input through the user interface (Figure 1) is stored on the web-server. ASP then converts that data into a text file formatted as an ANSYS parameter file which the macro can read.

SOLUTION – The initial intent was to run the ANSYS macros and solution directly on the web-server, but it was discovered that IIS simply cannot spawn a job as resource-intensive as Ansys. This was confirmed by Microsoft support engineers. The solution for this problem turned out to be somewhat more complex, but also more elegant than the original intent. Windows NT has the ability to call a remote shell program on a UNIX machine anywhere on the network. So, the ASP was written to utilize this ability and remotely run the ANSYS job on a much more suitable machine - a quad-processor UNIX workstation. This conserves the CPU on the server for hosting, and provides a faster solution for the Stabar FEA. Files are passed between the server and the workstation through FTP. This is very efficient because only small text files need to be transferred, such as the parameter file and the final HTML report. The relatively large Ansys database and results files stay on the workstation. Any future need to redirect the solution to an upgraded or different workstation can be met with minimal effort.

STATUS UPDATE – Keeping the user informed of the solution’s progress is another practical requirement of the ASP. This proved challenging to accomplish. The final solution is a status window that the ASP automatically rebuilds and refreshes every few seconds. It displays a text file periodically updated by the macro with messages such as “Meshing Complete”. Upon completion of the solution, the user is prompted to click a button on the status page that pulls up the final HTML report.

MULTI-USER CAPABILITY – The first version of the tool only allowed one user to interact with the tool at a time. Overwrite problems occurred otherwise. The current version allows for an unlimited number of design engineers to access the tool simultaneously. However, practical limits are set by the number of available Ansys licenses.

FUTURE PROJECTS – Theoretically, the same ASP logic could be applied to drive a variety of applications through a web-based user interface. Any engineering software with macro-like capability is a candidate. Accessing existing scripts, command files, and programs should be even easier.

CONCLUSION – The creative combination of technologies represented in the Web-Based Stabar tool significantly increases productivity for ArvinMeritor’s design engineers and FEA specialists. Furthermore, it provides a template for the development of similar analysis tools that provide the benefits of specialized skills and software to non-experts in the field of analysis. At ArvinMeritor, similar tools supporting other products are already under development.