Knowledge Base

Article Number: 12 | Post Date: August 17, 2016 | Last Updated: February 24, 2018

Where can I find the collision PDOF?

This feature was first made available as of the 2016-08-16 update. 

There are a couple of different 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 forwards 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).

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.

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. 

Tags: Principal direction of force, PDOF, force direction, impulse direction.

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