Chapter 14 | Staging Bicycle & Motorcycle Impacts
Introduction
Virtual CRASH provides the user a fast and innovative way to model motor vehicle accidents. It is particularly useful for studying pedestrian, motorcycle, and bicycle accidents due to the speed with which an accident scenario can be set up and simulated. In this write-up, we show how to stage a bicycle impact scenario in the Virtual CRASH software environment. These same lessons are applicable to setting up motorcycle collisions as well.
Create a New Scene
First, it is advisable to start a new project (Project > New) in order to ensure all objects are clear and that the Virtual CRASH simulation environment has been completely reset.
Create a Bicycle Object
Now that the scene has been reset, we can start our project. First let’s start by creating a bicycle object. Go to the left side panel and left-click the “+” symbol directly to the left of “motorcycle.”
This will expand the menu of motorcycle objects available to choose from.
Now press the “+” sign to the left of “Bicycle” to expand the list of bicycle objects.
This should reveal two bicycles to choose from: “Road bike” and “Standard M.”
In this example, we will choose the “Road bike” object. Using the left mouse button, hover the cursor over “Road bike” and press and hold down the left mouse. Continue holding the left mouse button, as you drag the cursor into the environment editor window. Once in the environment editor, release the left mouse button to “drop” your bicycle into the scene.
You should now see your bicycle object in the scene.We’re going to keep the bicycle aligned with the global x-axis (yaw = 0 degrees) to take advantage of the Group tool.
Modify the Bicycle’s Properties
Now that your bicycle is in the environment and is currently selected as indicated by the red box around its 3D shape, you can modify its properties as needed for your case. Click on the “edit” button in the lower left corner of the left control panel. Clicking this will reveal the selected object’s properties, such as position, orientation, and inertial properties.
You can reveal each of these properties by clicking on the “+” to the left of the label of each property (red box shown below).
You can scroll up through the properties list by using the narrow scroll bar (blue arrow). You can also use your mouse’s scroll wheel to perform the same task. You can press and hold the left mouse button while hovering the cursor over the “properties” title (at the top of the red highlighted box in figure above) and drag the mouse upward to move the properties menu up within the left panel. The figure below shows the same properties panel after moving the properties menu up to reveal all of the editable properties.
To modify any of the desired properties, simple click on the “+” symbol to the left of the property to reveal specific values. You can click within the specific text box and use your keyboard to change value,
or you can press the down arrow to the right of the input value to access a scroll bar which will let you scroll through values.
Insert vCRASH Man Human (multibody) Model
Now that your bicycle has been placed into the scene, we can place our human object into the scene as well. Press the “Vehicles” button on the left side control panel to access the vehicle database.
Next, using the thing scroll bar, or using your mouse’s scroll wheel, scroll to the bottom of the vehicle list until you see “multibody” and press the “+” sign to reveal your options (see next two figures below).
As in the case with the bicycle, click, drag, and drop your human model into the environment.For now, we’re going to pose the rider facing the global x-axis direction, aligned with the bicycle’s heading. This will allow us to use the Group tool.
Note Virtual CRASH has both a male and female human model.
With the human model selected (as indicated by the red box around the 3D model), you can again access and edit specific properties of the human model object by pressing the “edit” button on the bottom left of the left side control panel.
Position vCRASH Man on the Bicycle
Now it is time to place our human model on top of the bicycle. First, it is recommended to turn off the physics simulation during this process to make the positioning process easier. This is accomplished by left clicking on “Create” on the top control bar, then left clicking “Physics”, then left clocking “Stop Simulation.” This will shut off the physics simulator.
Next, switch to the four pane view to facilitate careful placement of our human model. This is accomplished by left clicking on the view selection icon on the bottom control bar, then left clicking on “View 4 symmetric.”
Your view should now appear as depicted in the figure below.
You can give each pane a unique appearance, depending on your preferences. The active pane is indicated by the red box surrounding it. To change panes, simply hover your mouse over the desired pane and left click. You should see the red box move to your selected pane. To modify the render or display type, go to render type icon on the bottom control panel, and select your preferred view.
Next, select your human model by first ensuring your mouse cursor is set to “Select, Move And Manipulate.”
Once you see the arrow cursor, you may simply hover your mouse over your human model inside of your active pane, and left click to select it.
With your human model selected, press the “edit” button on the bottom left to reveal the human model’s properties.
Next left click on “misc” and then left click on the “X” symbol appearing next to “auto align to plane.”
Turning off auto align to plane will allow you to position the human model in arbitrary positions without Virtual CRASH automatically repositioning the model to contact the xy plane.
Now select the top-down view pane, by hovering your mouse cursor over that pane and left clicking. Zoom in to get a closer view of your bicycle and human model by using your mouse’s scroll wheel.
You can fine tune your view by panning the scene within each pane. Simply move your mouse to an area within the scene away from any objects until you see the hand icon. Once you see the hand icon, you can left click within your scene and move pan the camera view around by holding the left mouse button and dragging side to side or up and down.
In the top down view, select your human model, again using the “Select, Move And Manipulate” cursor.
Select the human model, and press and hold the left mouse button. As you’re holding the left mouse button, position the human model over the bicycle seat.
Do this for all three views until you are satisfied with the relative alignment.
Note that because you turned off the physics simulator before overlapping the human model and bicycle model, there is no apparent interaction between the two.
Pose vCRASH Man on the Bicycle
Now we are going to use one of the many poses that come with Virtual CRASH, to give our human model a more natural riding position. Select the human model, and press the “poses” button in the left control panel. This will reveal a number of predefined pose positions for the human model. You can use your mouse’s scroll wheel to look through the list.
Left click on the word “cyclist” in the poses menu. You should see your human model now automatically posed in a bicycle rider’s configuration.
Now, again fine tune your human model’s relative positioning with respect to the bicycle to your satisfaction using the “Select, Move And Manipulate” mouse cursor.
Refine vCRASH Man’s Pose
Now we’ve positioned our human model atop of the bicycle. We could stop here and continue on with building the collision event, or we can fine tune our human model’s pose a bit more. To refine our model’s pose, we’ll need to select the joint manipulator tool. To get access to the joint manipulator, first select your human model, then left click on the cube on the top tool bar, then left click on “Joints.”
You should now see something resembling the figure below.
Using the “Select, Move And Manipulate” mouse cursor, left click the yellow shape at the hip of the human model. This should cause that joint to turn red, indicating that it has been selected for editing.
We would like to change the articulation of angle of this joint to bring the upper and lower torsos closer together. Let’s restrict the mouse’s control to only allow rotation about the Y-axis. Do this by selecting “Restrict To Y (Rotation)” as shown below.
Now use the angular rotation grip by hovering the mouse cursor over the grip, pressing and holding the left mouse button, and dragging the mouse so that the articulation angle is modified (red arrow in figure below).
Note this process can be completed iteratively over all joints until you are satisfied with the relative positioning of each of the body segments. Once you are satisfied with the relative positioning of the joint, change your selection type to “Object.” You should notice the yellow joint indicators have disappeared, and now a single box encloses the human model, with the Y-axis rotation grip still visible (see below). This grip will now rotate the whole body.
Rotate the whole body until you are once again satisfied with the orientation of the human model. Reselect the “Move and Rotate” control in order to fine tune the position of the human model (see below). Note, rotating the whole body will realign the body’s local x-axis such that it is not parallel with the global x-y plane. The initial velocity vectors of all objects point along their local x-axis direction, so one must correct the body’s initial vnz value to correct for any initial pitch rotation (see http://www.vcrashusa.com/kb-vc3-article24). If the correction is not done, the body’s initial velocity vector will not be parallel with the global x-y plane.
The figure below shows a depiction of the final positioning of the bicycle and rider using this iterative approach. This will be used for the pre-impact configuration (just prior to impact) in our simulation.
Now let’s group the bicycle and rider together for ease of simulation. This is accomplished by left clicking on the bicycle, then holding ctrl on the keyboard while simultaneously left clicking on the rider. This should select both objects as shown below.
With both object selected, go to Group > Group using the top toolbar.
This will now create a single group object, allowing both objects to be modified simultaneously.
Note, a Group has its own local x-axis; however, unlike other objects in Virtual CRASH, a Group’s initial velocity is aligned with the global x-axis. As the Group’s orientation (yaw, pitch, and roll) is changed from (yaw=0, pitch=0, roll=0), its vni and vnz values are automatically adjusted to give the desired initial velocity vector direction, but only if the initial speed is first set to a value greater than 0. Since both the bicycle and rider were facing along global x-axis direction when the Group object was formed, they also will also face along the Group’s local x-axis direction which starts at yaw = 0. Once the Group object is created, the Group’s velocity and orientation can then be modified like any other object, and the bicycle’s initial orientation and velocity vector, as well as the rider’s, will properly reorient accordingly as the Group is modified, but again, only if the Group’s initial speed is first increased from 0 first. After the initial speed is increased from 0, then the Group’s orientation can be modified. If the Group’s initial speed is 0, and the orientation is first modified, you may see unintended behavior. You can, of course, expand the Group’s folder and set the initial velocity and orientation of the rider and bicycle separately.
Insert Vehicle into Scene
Repeating the same process that we performed at the start of the lesson, let’s insert a car into our scene. We can orient the car by first ensuring our mouse cursor type is on “Select, Move And Manipulate.” Left click on the car to access the fast control icons. Hovering your mouse over various parts of the car will let you see the Move or Rotate controls. You can also right click on the car to bring up the fast edit dialogue box to control position and orientation.
Let us now turn on the physics simulation by once again selecting Create > Physics > Stop Simulation. Since the physics simulation had previously been stopped, this will reactivate the simulation.
You may set the initial speeds of both vehicles and run your simulation as you would for any other crash simulation (see below). If you’re using a Grouped rider and bicycle, first set the initial speed of the group to a value greater than 0 before reorienting the Group from (yaw = 0, pitch = 0, roll = 0) to ensure proper behavior.
Note, if you do not group the bicycle and rider together, when setting the initial velocity of the bicycle and rider system, take care to ensure that the rider’s vnz component is properly set to account for initial pitch adjustments (see http://www.vcrashusa.com/kb-vc3-article24).
Add Kinematics
The “Add Kinematics To Selection” tool is a very convenient method to properly position vehicles along a pre-impact trajectory such that the physics simulation starts just at the end of the kinematics path. This is ideal for bicycle and motorcycle impacts, where the human models effectively act as flaccid bodies. The bodies will remain locked in the pre-impact configuration during the kinematic trajectory portion of the motion, then will be allowed to act according to the physics simulation at the end of the kinematic trajectory. To use the kinematics tool, simply left click the object, and left click to Create > Physics > Add Kinematics To Selection.
Do this first for the car object. For the bicycle+rider object, you must expand the group folder on the left control panel to, then using ctrl+left click, select both the rider and bicycle objects.
With those two objects selected, go to Create > Physics > Add Kinematics To Selection as you did for the car.
You should now see two pre-impact trajectories attached to the initial position you chose for your simulation.
By left clicking on the boxes within the kinematics trajectories, you can control the positions of the vehicles along these paths.
You may need to fine-tune the angular orientation of the bicycle+rider group along the pre-impact trajectory. You can impart some roll angle into the pre-impact trajectory here for a more natural look to your animation. Left click and select the rider+bicycle group, then right click to access the fast control dialogue box to modify these parameters.
Do this for the first and second positions on the Kinematic trajectory.
Alternative Workflows in Forward-time Evaluation
Instead of initiating your simulation at or near the point of impact, Virtual CRASH provides enhanced workflows that enable the rider to stay on or inside the vehicle for a significant period prior to the collision. The details of these workflows are outlined below.
Attach RIGID mesh rider to vehicle
With a multibody selected, go to tools > convert.
The option “to mesh” freezes the multibody into a solid mesh object. All joints are removed as well as rigid body properties. One use of this feature is to merge a multibody shape with a vehicle. For example, below, we converted the multibody to a mesh. Then, we removed physics from the scooter and grouped together the multibody and scooter meshes. We then made the group into a rigid body object, added front and rear axles, and modified the wheel data to match the original scooter’s data. We then increase the roll and pitch moments of inertia to 1e6 to ensure the scooter+rider system remain upright as it moves forward (see below). Using this same method, a stationary occupant can be placed in an occupant cabin of a vehicle model for a more realistic looking visual aid.
CONVERT MULTIBODY TO RIGID BODY OBJECTS
Multibodies can also be converted to “rigid bodies.” When this option is selected, Virtual CRASH will build a multibody-like object from hyperellipsoids and ragdoll joints which are placed in an identical pose as the original multibody object. A new group object will be placed in the project menu consisting of the hyperellipsoid objects, joint objects, and the multibody skin “envelope”. In the figure below, the envelope is hidden and joints revealed.
We can now connect the rigid body objects to the scooter or other rigid body objects using the joint tools. In the case below, we attach the hands, feet, and pelvis to the scooter. Here we give the scooter a third axle with small wheels in order to move forward and remain upright.
The joints can then broken at the moment of impact so the rider can move independent of the scooter.
Note, the rigid body object created with the “to rigid body” feature cannot be posed in the same manner as the parent multibody object after creation. The multibody global weight and size input functions are also lost, though individual hyperellipsoid and joint properties can be configured. The rigid body object is also not optimized to deliver fast simulation results as the parent multibody object is, and therefore you may experience increases in CPU time needed to complete simulations using these objects. You may also see differences in simulation scenario outcomes where the rigid body object is used rather than the parent multibody. Note, “auto align to plane” and “optimize” are also unavailable for the rigid body object. See the links below to learn more about this workflow.
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