The driver model follows a path (blue dashed line) that differs from the reference line of the 3D road. Click for more detail.

BikeSim, CarSim, and TruckSim use custom vehicle math models. Some VehicleSim model features exist in all three products.

This page describes VS Paths and Roads.

Paths and Roads

BikeSim, CarSim, and TruckSim include multibody models that simulate vehicle dynamics under driver/rider control on a 3D ground surface. This 3D surface is a major part of the environment in which the vehicles operate.

The vehicle math model and other moving objects are controlled with use of 2D references paths. Originally provided for the use of internal driver or rider models, VS Reference Paths are now used in ADAS scenarios to specify motions of other objects, such as traffic vehicles and pedestrians.

VS Reference Paths

A VS Reference Path is a continuous line that exists in a horizontal plane with continuity in position and gradient. That is, there are no sharp corners.

Besides providing a simple means for specifying motion of a simulated object, the path defines a 2D coordinate system for describing locations that are near the path. The path coordinates are station S (distance along the path) and lateral coordinate L (distance a point is from the path, measured on a line that intersects the point and the path, and is perpendicular to the path at the point of intersection.

VS Solvers support up to 50 Reference Paths in memory at the same time. Existing paths can also be redefined any number of times during a simulation, such that there is really no limit to the number of paths that can be used.

The main applications for the S and L coordinates associated with a Reference Path are:

1. The VS Driver and Rider models work by reducing the absolute value of the L coordinates of preview points ahead of the vehicle relative to a target path.
2. Objects (traffic vehicles, targets for on-board sensors, etc.) can be located relative to the path using S and L.
3. Elevation and friction for a road surface are defined with S and L coordinates associated with specified path. This allows a great deal of data density in the vicinity of the path, without requiring information in other areas.

The path coordinates are station S (distance along the path) and lateral coordinate L (distance a point is from the path, perpendicular to the path).

VS Solvers include a Configurable Function LTARG that defines a lateral offset L as a function of S. This function is typically used together with a Reference Path to define variants of the Reference Path. For example, specify a lane-change maneuver relative to the the Path that defines the centerline of a road. As another example, define multiple lanes relative to a road centerline. Up to 100 LTARG datasets can loaded in memory at any time.

A VS Reference Path is a sequence of contiguous segments. For example, the path shown to the right could be defined with three segments: a straight line going from point 1 to point 2, a circular arc going from point 2 to point 3, and a straight line starting at point 3. Three types of segments are supported:

1. Straight lines.
2. Circular arcs, defined either with radius or curvature (inverse radius).
3. Spline functions specified with tables of X-Y coordinates.

The path is built from segments starting with an initial global location (X and Y coordinates) and heading angle. Each segment is defined with a type and one or two descriptive parameters. In assembling the path, the VS Solver maintains continuity, such that the starting location and heading of each new segment match the ending location and heading of the previous segment.

The 3D road model uses a Reference Path and several Configurable Function dataset to define elevation near the path. Click for more detail.

VS Road Model

The VS road model, introduced with CarSim 5.0 in 2001, defines elevation geometry and friction using S-L coordinates associated with a VS Reference Path. This allows detailed information to be concentrated in the region of interest (i.e., on or near the road path).

The 3D surface is defined as a collection of up to 50 road surfaces, where each road surface is in turn defined using a 2D VS Reference Path.

The properties of each surface are defined with Configurable Functions of S and L. These include friction and two components (datasets) for elevation. There is also a Configurable Function that defines a component of elevation as a function of only station.

Connections between adjacent surfaces are specified to allow the simulation to automatically move points from the moving bodies (tire contact points, moving object locations, etc.) between surfaces automatically. Alternatively, VS Commands can be used to switch from one surface to another, or to redefine the properties of the surface that is already resident in memory.

The VS Browser uses the road datasets to automatically generate animator shapes to allow visualization of the ground geometry and its relationship with vehicle motions in animations.

Road Roughness

Roughness profiles allow realistic road roughness inputs to be used in simulated tests without requiring the extensive amounts of data needed to fully map a 3D surface, especially in cases where the vehicle is not following a straight path. These can make use of data routinely measured by highways agencies, or generated with suitably filtered noise from random number generators.

A "wandering profile" is defined as an increment of elevation Z defined as a function of distance travelled, where the distance travelled is calculated in the math model by integrating the forward speed for the front wheels, and adding the distance between axles to get the proper delay of inputs for other axles. CarSim and TruckSim models include two wandering profiles; BikeSim includes one.