Movement along a Path

The current path position is indicated as a red sphere on the path (if the path position display is enabled). The movement of the sphere along the path can be controlled at any time during simulation by adjusting the target nominal velocity parameter, which is the desired nominal velocity of the sphere along the path. The sphere will then adjust its current nominal velocity by adopting a constant acceleration until the current velocity matches the desired velocity as can be seen in following figure:

[Simple path velocity control by adjusting the target nominal velocity parameter]


This represents an easy method to get the path position moving or stopping with a given acceleration value.

There are however situations where one would like to see a path velocity reduced when passing certain specific path positions (e.g. when the path describes a tight 90 degree direction change, one might want to reduce the velocity by 50%). The user could handle those situations manually by adjusting the target nominal velocity accordingly. There is however a more convenient way, which is to apply a velocity profile to the path: each control point has a relative velocity value assigned, that can be adjusted when in the path edit mode. That relative velocity value tells the simulator that it should adjust the nominal velocity and the target nominal velocity when the path position is close to the control point. The relative velocity value of a control point is applied to its Bezier points in an interpolated fashion. Following figure illustrates the velocity profile concept:

[Velocity profile of a path]


When the relative velocity profile is not a constant value at 1, the nominal velocity is modified to take into account the relative velocity profile. The resulting velocity is called the shaped nominal velocity. Following figure illustrates the operation that combines the relative velocity profile and the nominal velocity:

[Relative velocity, nominal velocity, and the resulting shaped nominal velocity]


The user can manually adjust the relative velocity value for each control point (when in path edit mode), or an automatic velocity profile can be selected. Clicking adjust velocity profile in the path dialog displays following parameters:

[Path velocity profile dialog]


  • Turning circle diameter for 50% velocity: a turning circle diameter for a 50% velocity reduction can be enabled; use this option to limit the velocity at which the path position movement direction changes (movement direction is different from orientation!).
  • Relative velocity at rotation axis change: a relative velocity when a rotation axis change is detected can be enabled. This option is a little bit more complicated to understand: the instantaneous rotation axis is the vector that brings a reference frame from one orientation onto another. A rotation axis change is detected when the angle between current instantaneous rotation axis and previous instantaneous rotation axis is larger than 60 degrees. It is important to understand that the instantaneous rotation axis has nothing to do with the trajectory position or the rotation velocity of the orientation component. It is a measure on how abruptly the orientation variation changes.
  • Zero velocity at end-points: most of the time the user wants to have zero velocities at an open path's end-points. This can be manually adjusted by setting the first and last control point relative velocity value to zero, or the simulator can take care of that automatically if this check-box is enabled.
  • Relative acceleration: this parameter is the maximum relative acceleration tolerated to generate the velocity profile. This parameter is a relative parameter!
  • To actually make an object move along the path, you will first need to attach the object to a dummy, then assign the dummy to follow the path position (with an optional offset). Refer to the section on dummies for more details. Alternatively, you can have an object follow a path by using the simFollowPath API command.


    Recommended topics

  • Paths
  • Path control points and Bezier points
  • Path position and length calculation method
  • Path dialog
  • Path edit mode
  • Dummies