Identification of the Problem

Vehicle designers have used commercial CAD packages such as Pro/E, which uses an Advanced Boundary Representation (BREP) to model solid geometry. This has required that the geometry be converted to BRL-CAD format; yet converting from BREP to “Faceted BREP” is time consuming and labor intensive, while producing results that are not exact. In addition, the manual re-creation of targets in CSG has also been a very lengthy process. Furthermore, in the “Faceted BREP” system, all faces are planar, (i.e., polygons, triangles, and/or quadrangles); and the boundary only approximates curved solid geometry.

Solution

RtCAD is ThermoAnalytics’ new ray-tracing tool for MUVES. This tool directly interrogates 3-D geometry models created by commercial CAD systems like Pro/E. 

This innovative system utilizes the Pro-E or STEP surface representation of solid geometry (BREP), rather than the converted mesh format (“Faceted BREP”).

Background

State-of-the-art vulnerability and survivability codes simulate the path of the threat projectile through the target using a ray-trace calculation. The simulation launches a threat projectile towards the target according to a user-specified pattern of shots. Associated with each ray trace are parameters associated with the threat (threat type, mass, velocity, etc.). Using the starting point and direction vector of the initial ray, the program traces that ray until it intersects with the target. The main analysis routine tasks the ray-tracer routine with finding the intersecting (entry) point, the angle with which the ray intersects with the surface, and information about the target component that was struck. This information includes component name, material properties, surface thickness, and curvature. The main routine passes this data, along with the threat parameters, to the appropriate Interaction Module (IM). The IM calculates the effects of the physical interaction between the threat and the component including estimating what level of damage the component incurs. The IM can affect how the solution proceeds: The ray may be deflected (which results in a nonlinear path for the projectile through the vehicle) or new threats may be generated (e.g., spall). Another possibility is that the IM will decide that the projectile lacks sufficient energy or momentum to penetrate farther into the vehicle, in which case the code stops the ray trace. See Figure 1 below.

Figure 1:  MUVES Ray Trace Procedure

The BRL-CAD ray tracer employed by MUVES predicts accurate line-of-sight (LOS) ray-trace information when given the exact mathematical descriptions of vehicle components. Although conversions do exist from some commercial CAD formats to BRL-CAD, they are not usually exact. For example, the Pro-E to BRL-CAD conversion uses a faceted description. This type of geometry translation must use an overwhelmingly large numbers of facets to describe Pro-E solids accurately; yet, the resulting boundary representation is still an approximation. The alternative is a painstaking manual conversion of Pro-E geometry to BRL-CAD. Due to the expense, the analyst can perform a manual conversion only periodically during the development of a vehicle—perhaps only once—after the vehicle design is finalized. Accordingly, survivability assessments only have limited impact on the design of the vehicle.

MUVES was designed to accommodate additional ray tracers by maintaining a clean separation between the vulnerability estimation routines and the ray tracer. Support for a new geometrical description (i.e., Pro-E) is added by implementing a relatively small wrapper code that translates MUVES ray-tracing calls to those of the underlying ray tracer. The primary outputs of the ray-trace routine are entry and exit points, surface vector normal, surface thickness, and impactor/target obliquity, as illustrated by the sample ray trace in the following Figure 2.

Figure 2: Schematic of a ray trace.

RtCAD, the ra tracer developed for directly interrogating Pro-E and STEP solid geometry models, uses the PTC GRANITE geometric modeling kernel. GRANITE library functions are called to read Pro-E and STEP files and for finding ray-geometry intersections.

In addition to finding efficient algorithms for intersecting rays with surfaces and solids, much of the science of ray tracing has focused on reducing the number of ray-geometry intersections that must be examined. RtCAD uses a technique known as spatial subdivision, in which the bounding box of the vehicle to be ray traced is divided into volume elements, or voxels. RtCAD uses uniform spatial subdivision, in which all voxels are the same size. Before ray tracing, the geometry is preprocessed to create a list of all faces (the surfaces and boundaries that make up BREP solids) that occupy each voxel.

Modeling Overview

Preparing a Pro-E model for use with MUVES requires the following steps, which are in addition to or different from BRL-CAD:

1) Simplify the Geometry; see the top figure to the right.

Ray tracing BREP solid geometry is an inherently more complex problem than ray-tracing equivalent CSG representations. The BREP description of a simple shape is composed of numerous planes and a cylindrical surface that are bounded by (segments of) multiple lines and circles. To find the intersections of a ray with the equivalent BREP, the ray must be intersected with (a subset of) the planes and cylindrical surfaces. Each potential intersection point must be tested; this is a relatively expensive two-dimensional calculation.

In the CSG System (used by BRL-CAD):

• “Primitive” solids form basic building blocks;
• Complex solids are formed by applying Boolean operations (union, intersection, difference) on (primitive) solids; and
• Ray tracing is very efficient (order of magnitude faster than BREP).

In the figure to the right, geometry is not required in the MUVES analysis. In this case, bolts and the corresponding holes are removed by “suppressing” them, a step that requires the “part history.”

As with BRL-CAD, the interior space of a Pro-E or STEP geometry will usually be modeled so that it contains continuous regions of air. This is done in Pro-E or the CAD program used to create the STEP geometry by adding parts that explicitly represent air “compartments,” such as the air within the crew compartment, the engine compartment, etc. The added parts are designated as air parts when the MUVES region map is created.

3) Convert Pro-E (or STEP) geometry to "GRANITE" Format (Standalone Utility).

Reading either a Pro-E or STEP file is an expensive operation due to the parametric information (Pro-E) or exchange information (STEP). For this reason, a stand-alone translator is provided to convert Pro-E or STEP geometry to an efficient, compact format suitable for ray tracing ("GRANITE" format). The conversion is performed once and can then be used for multiple MUVES analyses.

4) Create a MUVES Region Map.

The MUVES region map file creates an association between parts in the geometric model and MUVES components. A utility was developed to help automate the creation of the region map file. See the figure to the right. The assembly/part hierarchy shown in the left pane of the region map editor will match exactly the hierarchy shown in Pro-E or the CAD program used to create the STEP geometry. MUVES components are added to the right pane of the region map editor, and the association between parts in the geometric model and MUVES components is made by moving assemblies/parts from the left pane to the right.

Technical Outcomes

Accurate Analyses

A shot-line-by-shot-line comparison was made between the results obtained with BRL-CAD and Pro-E representations of the same geometry. Cell plots produced by the output of the two analyses were compared. See the following diagrams.

BRLCAD Ray Tracer

RtCAD Ray Tracer

Shot-lines 695 & 701

The STEP shot liner appears to miss the sprocket_left and sprocket_right components while the BRLCAD shotliner hits them. When examining this shot, it appears that the shot-line hits are very near the edge on the two cylindrical components. This is likely due to round-off errors and/or minor differences in shot-line intersection routines.

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RtCAD Run Time Magnitude

• For a simple tank with realistic (simplified) track & 2Ghz P4 CPU
• Pro-E to Granite conversion, the time is the same as loading a Model in Pro/E.
• The MUVES analysis 10000+ rays takes 30 seconds. MUVES / RtCAD may not be as fast as MUVES / BRL-CAD, but it should be fast enough for most analyses. See the figure to the right.

A Direct Interface With MUVES

CAD Support

With RtCAD, there is also broad CAD vendor support:

• AutoCad
• CATIA
• MSC/Patran
• Pro/Engineer
• SDRC Ideas
• SolidWorks

Pro-E / STEP models may need to be reduced in complexity to that which is typically used for vulnerability analysis. However, simplifying CAD models is much less labor-intensive than converting to BRL-CAD. The use of low-cost Linux clusters or available high-performance computers can also be considered.