Article Number: 61 | VC4 | VC3 | Post Date: May 9, 2018 | Last Updated: May 9, 2018
Is it possible to apply a pre-defined crash pulse to a vehicle?
Yes, using the force tool, you can apply forces to rigid body objects over a specified time interval. You can choose between preset shapes such as rectangular or sine function pulse shapes, or you can draw your own shape.
Suppose we want to apply a crash pulse to the rear of our vehicle to simulate a 5 mph Delta-V rear impact collision.
Left-click on Create > Physics > Force to select the force tool.
Next, hover your mouse cursor over the rigid body object to which you wish to apply a force. Note, you can apply forces to any rigid body object, including multibodies. The object will turn light blue, indicating the force will be applied to it once you left-click. Note, the arrow displayed while hovering the mouse over the object indicates the direction normal to the polygon the mouse is hovering over. The force vector will initially be oriented anti-parallel to this normal axis.
Now, left-click on the object, near the desired point of application.
Note, you can continue left-clicking to apply additional force vectors. Right-click to terminate the command. The time of application for each force vector can be separately input.
Next, fine-tune the placement of the force vector using your mouse or using the position-local and rotation-local inputs. Note, the position-local and rotation-local values are fixed within the vehicle’s reference frame. So, a force vector’s orientation will remain fixed throughout the simulation with respect to the vehicle’s orientation. A position-local of (0,0,0) for vehicles will correspond to (x,y) position at the vehicle’s geometrical center and a z position of the lowest polygon on the vehicle mesh. For rigid body shapes made from primitive 3D shapes such as boxes or spheres, (0,0,0) will correspond to the true geometrical center.
Next, in the timing menu, set the time interval for the application of your force function. Here we’ll simulate a rear impact as occurring over a 120 msec time interval, beginning at 0.1 seconds into the simulation. Note, the “from” and “to” values are specifying the simulation time at which to start (“from”) and end (“to”) the force function.
Square wave pulse
Now, with the time interval set, to approximate a square wave pulse, simply input the force in the “strength” input field (units are Newtons). This will apply a constant force value, but only during the specified time interval. A square wave acceleration pulse is shown in the acceleration-x graph below.
Note, the observed spikes above are caused by impulse interactions between the multibody driver and the vehicle model, as the vehicle’s seat pushes the multibody forward. Removing the multibody, we see a perfectly clean pulse.
For a square wave pulse, impulse-momentum can be used to provide an estimate of the force required to obtain a target Delta-V, simply given by:
F = m x Delta-V/Delta-t
(assuming the force vector is directed through the vehicle CG). In this case, the mass is the total mass of the driver and vehicle. The force can then be fine-tuned using your mouse scroll wheel as needed to obtain the target Delta-V value. By monitoring the time-velocity graph, one can quickly see the obtained Delta-V for each change in “strength” input.
Sine wave pulse
Left-click on the box next to “strength” to access the dropdown menu to specify the function type to use for the force tool. Here we left-click on “sin”. The diagram tool will open, which lets you adjust the various sine wave parameters. Here we set the offset x value to 0.1 seconds since this is the starting time for our force. Our period is set to 2 x 0.12 seconds since we are only interested in using a half sine wave. The amplitude is then adjusted such that we obtain our target Delta-V. For a half sine wave a reasonable first estimate is to use
F = m x Delta-V/Delta-t x (Pi/2).
The amplitude is further adjusted as needed.
Inspecting the time-velocity graph, we see the charactheristic turn-on of the integrated sinusoidal acceleration.
Other force function options
You can also draw pulses by hand using the free hand option. Decrease your simulation time and increase the number of segments. The maximum allowed number of segments is 200. You can use the “Select And Move” option to drag the points up and down in the diagram tool, or use the “Set Points” option to left-click in the diagram window.
Force functions can also be specified using the rectangular pulse or intervals option.
The force tool is a very powerful tool that can be applied in many types of cases. Here we see the force tool applied to simulate the occupant kinematics of a frontal impact.
By first converting your multibody to a rigid body (http://www.vcrashusa.com/guide-chapter20/#Multibody), you can even apply the force tool directly to multibody objects.
Tags: force function, impulse function, biomechanics, injury, force specification, acceleration pulse.
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