Knowledge Base

Article Number: 39 | VC4 | VC3 | Post Date: September 14, 2017 | Last Updated: September 14, 2017

## Can one elevate objects in Virtual CRASH at a user specified speed?

Yes. In Virtual CRASH 3 and Virtual CRASH 4, there are several ways to elevate objects.

First, let’s start with a simple case. Suppose we would like to elevate this cylinder object in order to create a visual aid of a barrel being lifted.

If we do not care about its interactions with any other objects, or about accessing its (x,y,z) versus time report data, we can simply use the diagram tool to specify the object’s (x,y,z) coordinates versus time. To access the diagram tool, left-click on the square next to the coordinate you wish to specify. In this case, we’re going to specify the z coordinate versus time. Here we’ll choose the intervals option (also shown here: http://www.vcrashusa.com/blog/2017/2/11/building-complex-roads).

The intervals option allows you to select the number of key positions (“count”) you wish to use, assuming your coordinate starts at the “offset y” position. In this case, since our barrel began at z = 2 ft (geometrical center was 2 ft above the ground), our offset y is 2 ft. We then specified 4 key positions (the white dots in the graph). These positions can be moved up or down or left and right within the diagram window just by left-clicking and dragging.

Here, we reduced the number of intervals to 2 and positioned the key points so that during interval 1, the barrel ascends at 1 ft/s, and in interval 2, the barrel ascends at 1 ft/3 secs.

The position versus time can be confirmed via visual inspection, however, non-rigid body objects do not forward their position data to the report.

If we wish, we can convert this to an unyielding object by going to Physics > Make Unyielding / Terrain from Selection, which will allow the barrel to interact with other rigid body objects. The position versus time can be confirmed via visual inspection, however, again unyielding/terrain objects do not forward their position data to the report.

Here for example, our cylinder object is lifting the vehicle.

Now, suppose we would like to lift our car (or any other rigid body object) at a specified speed. Here we can use our unyielding barrel object to do the lifting. In our final animation, we could hide the barrel (or elevating unyielding object) from view.

In this case, a 3D model of a mechanical lifting system could be inserted into our environment to create a visualization of a vehicle elevating by a lift device.

Now that we have a rigid body object elevating in response to the unyielding object’s motion, we can create a report of the rigid body’s position versus time, whose displacement should match the hidden cylinder’s assuming the rigid body remains at rest with respect to the cylinder. With a simple analysis of the vehicle’s z position versus time, the cylinder’s velocity in the z direction is confirmed. Note because there is some slight motion with respect to the cylinder of the vehicle’s cg as it is being lifted due to contact forces, we see some small variation about the known vertical velocity.

Another option is to lift the vehicle by pulling up using joints or the rope tool. Here we’ve increased the offset of our cylinder object so that it begins over the vehicle. We’ll attach a series of spherical joints to connect the two. Recall, to connect objects by a joint, first select the joint type, then left-click on the connection point on your first object and hold, drag the mouse to the second object’s contact point and release the left mouse button.

Here we’ll use 3 joints in order to remove rotational degrees of freedom.

Next, we can stiffen the joint stiffness values of the spherical joints. Recall, the ball and socket is held together by a spring-dampener system. Increasing the spring constant helps to prevent oscillations.

Here we confirm the rate of elevation. Note the stable velocity curve, without fluctuations observed above, when using this approach.

We can also use the diagram graph display to visualize the vehicle’s speed versus time, which is isolated to the z-direction. Again, we see the expected behavior (note the graph is in mph).

We can also use the rope tool (see http://www.vcrashusa.com/guide-chapter19):

To make the rope tool more chain-like, we increase the joint stiffness:

In the example below, we’ve broken our crane 3D model into the main vehicle, the boom, and the hook. Each are separate rigid body objects. We’ve reconnected the boom to the vehicle using a hinge joint (see: http://www.vcrashusa.com/blog/2016/7/3/making-rotating-assemblies-with-a-hinge-joint). Again, using the diagram tool, we can set the articulation angle of the hinge as a function of time.

We’ve also used a hinge joint to connect the hook to the boom, and the rope tool to connect the hook to the cargo container.

Tags: Lifting objects, elevating, diagram tool, rope tool, joint, crane, how to lift, how to elevate.