Article Number: 12 | VC6 | VC5 | VC4 | VC3 | Post Date: August 17, 2016 | Last Updated: July 27, 2022
Where can I find the collision PDOF?
There are a few ways to access the principal direction of force (PDOF) value for a collision.
Note, here PDOF is defined with respect to the impulse vector tip not tail, with 0 degrees directed forward and 90 degrees to the left. This convention means that a frontal collision with a rearward directed force has a PDOF of 180 degrees. To switch to SAE conventions, you'll need to flip the Virtual CRASH PDOF sign (to account for the difference in y-axis orientation for SAE conventions) and add 180 degrees to define the PDOF as the angle between x-axis and impulse vector tail rather than tip (see graphic below).
PDOF from Instantaneous \(\Delta \vec{v}\)
The instantaneous \(\Delta \vec{v}\) is the change-in-velocity that occurs due to what is effectively an instantaneous impulse exchange (over one integration-time step). This for example, occurs when the Kudlich-Slibar impulse model exchanges impulses between vehicles at the moment of impact. This differs from a cumulative \(\Delta \vec{v}\), which is the change-in-velocity that occurs between two arbitrary moments of time.
Let's start by looking at the t-bone collision example below.
By selecting the auto-ees object in the left side control panel, you can access the input data for the Kudlich-Slibar collision model, as well as see resulting output data. By left-clicking on "previous contact" under the selection menu, you can ensure that you are selecting initial or primary impact. Below we drew in the black line in the x-y plane to quickly estimate the impulse vector directions geometrically in the Global (Earth) frame. In this case, our black line has a yaw value of 38.8 degrees, implying the PDOF for the orange vehicle is aligned with the direction impulse ni = 38.8 degrees (rotation about the Global frame z-axis), whereas the PDOF for the blue vehicle is aligned with the direction impulse ni = 38.8 degrees - 180 degrees = -141.2 degrees.
Vehicle Accident Analysis and Reconstruction Methods
by Raymond M. Brach and R. Matthew Brach
The impulse vector direction can be accessed directly by going to the object 1 and object 2 menus of the ees object. There you will see "impulse ni" and "impulse nz". As indicated above, these values describe the direction along which the impulse vector is aligned for each vehicle, in Global (Earth) frame. "impulse ni (local)" and "impulse nz (local)" describe the impulse vectors within the local (vehicle) frames. Remember, the rotation angle about the Global frame z-axis is given by impulse ni, and the angle from the x-y plane is given by impulse nz. The rotation angle about the vehicle frame z-axis is given by impulse ni (local), and angle from the vehicle frame x-y plane is given by impulse nz (local). Of course, if you are trying to correlate your Virtual CRASH simulation results with EDR data, you will want to compare the local impulse directions to your PDOF implied by your EDR data.
In the case shown above, we see the object 1 impulse unit vector (blue vehicle) has components (impulse ni, impulse nz) = (-141.215 degrees, 0.607 degrees). For our orange vehicle ("object 2"), the impulse vector is pointing in the anti-parallel direction (impulse ni, impulse nz) = (38.785 degrees, -0.607 degrees). These values agree with our rough estimate above.
You can also access the object 1 impulse vector direction by using the Report tool. Scroll down the Report and locate the corresponding contact. You'll see the impulse magnitude and direction values.
PDOF from Cumulative \(\Delta \vec{v}\)
As of the Summer 2020 Software Update, VC4 and VC5 users can use the dynamics info tool to report the cumulative \(\Delta \vec{v}\). The cumulative \(\Delta \vec{v}\) is the change-in-velocity that occurs between two arbitrary moments of time, defined by:
$$ \Delta \vec{v} = \vec{v}(t_f) - \vec{v}(t_i) $$
where \(t_i\) is the initial time and \(t_f\) final time for the calculation. The cumulative \(\Delta \vec{v}\) may be preferable to instantaneous values in cases where contact interactions require numerous impulse exchanges to resolve, cases where tire forces may be non-negligible compared to collision forces, or cases involving joint couplings (such as tractor-trailers).
See this blog post to learn more about accessing the cumulative \(\Delta \vec{v}\).
Tags: Principal direction of force, PDOF, force direction, impulse direction.
© 2021 Virtual CRASH, LLC. All Rights Reserved