Joint Initial Velocity: Revolute

Format

<JointInitial_Rev
      joint_id    = "integer"
      iv          = "real"
</JointInitial_Rev>

Attributes

joint_id

The ID of the revolute joint whose initial velocity is being specified.

iv

The initial rotational velocity of the revolute joint. The value specified is the rotational velocity of the I marker of the joint relative to the J marker of the joint. The unit is radians per model time unit.

A positive value specifies a velocity in the counterclockwise direction.

Example

Consider the simple 2-dimensional pendulum shown in the image below.

Figure 1. Specification of Initial Velocity for a Revolute Joint

Assume that the pendulum body (shown in red) is pivoted at the global origin, denoted by point P, by a revolute joint, such that rotation is allowed about the global z-axis. The center-of-mass of the pendulum is at B.

At the start of the simulation, the pendulum is rotating counterclockwise about the global z-axis at 20 radians per second. The initial velocity of the revolute joint may be specified as follows:

<JointInitial_Rev
      joint_id  = "1"
      iv        = "20">
</JointInitial_Rev>

Comments

1. Besides joints, there are two other ways to specify initial velocity in MotionSolve models: using the Motion_Joint or Motion_Marker element and using the Body_Flexible, Body_Rigid, or Body_Point element.
2. If there is a conflict among initial conditions specified using these various elements, the following precedence rule is applied: Motion elements override the joint elements which in turn override the body elements.
3. The number of initial conditions should not exceed the total number of degrees of freedom in the model.
   • A system with one degree of freedom should have only one velocity initial condition. The constraints apply consistent velocities to the rest of the states in the model.
   • A system with zero degrees of freedom should have no velocity initial conditions.