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Suppose that you rotate a body by clicking the Object Control icon on the View Control toolbar. However, when you simulate the model, you find that the body starts to rotate from its initial position rather than the rotated position.
<Figure 1>
In Figure 1, the image on the left shows a cylinder that has been rotated 60 degrees counterclockwise using the Object Control function. Despite this, when the simulation begins, as shown in the image on the right, the cylinder starts to rotate from its initial position. This situation often occurs when modeling in RecurDyn.
In the above example, the Motion Type of the revolute joint is set to Displacement (time), as shown in Figure 2.
<Figure 2>
This settings means that the revolute joint is rotated 0 radians at 0 seconds and 1 radian at 1 second. Here, the rotation degree is the orientation of the action marker relative to the base marker. For the revolute joint (C1_RevJoint1) created in the cylinder on the right, the base marker is set to Ground.Marker2 and the Action marker is set to Body2,Marker1.
<Figure 3>
In the initial modeling state, the action marker is rotated 60 degrees. However, during the simulation, the cylinder rotates according to the motion conditions defined for the revolute joint. As a result, the cylinder's orientation at the beginning of the simulation (0 seconds) differs from the orientation defined during modeling. So, how can you start the motion with the cylinder rotated 60 degrees at the beginning of the simulation (0 seconds)?
- The first solution is to set the Expression setting to Time+60D in the Motion dialog window, as shown in the figure above. The D converts the value in degrees into radians. In short, this method edits the motion condition of the revolute joint to offset by 60 degrees at the start of the simulation(0 seconds).
- The second solution, which is more recommended, is to define an angle between the base marker and the action marker. If you rotate the base marker by 60 degrees, then the base marker and the action marker will have the same orientation in the modeling state. Then, when you start the simulation, the cylinder will start to rotate from its rotated position.
To rotate the base marker, perform the following steps:
<Figure 4>
Open the Properties window for the revolute joint. The Connector tab of this dialog window contains a Copy Action to Base button and a Copy Base to Action button. To apply the origin and orientation of the action marker to the base marker, click Copy Action to Base, and then click Apply.
