Force

The force element, in MBDyn, is a general means to introduce a right-hand side to the equations, i.e. an explicit contribution to the equations. There is a basic distinction between abstract and structural forces: the former apply to arbitrary equations, while the latter are specific to structural nodes and have a spatial description, essentially a direction in space which is amplified by a scalar coefficient that may depend on parameters, e.g. on time. The syntax of the force element is:

  <normal_arglist> ::= <force_type> , <force_arglist>

where force_type can be abstract for an abstract force, or conservative or follower for a structural force. The latter types also apply to the couple element, which can be structural only. It is discussed in this section because of its input syntax commonality with the structural force elements.

Output

The output is discussed according to the types of forces. The label of the element is output first in all the cases.

Abstract force

    <force_type> ::= abstract 
    <force_arglist> ::= (node_dof) <dof> ,
                        (drive_caller) <force_magnitude>

the dof field is a normal node_dof but no order is required since the force simply applies to the equation related to the node, regardless of the order.

Abstract reaction force

    <force_type> ::= abstract internal
    <force_arglist> ::= (node_dof) <dof1> ,
                        (node_dof) <dof2> ,
                        (drive_caller) <force_magnitude>

the dof1 and dof2 fields are normal node_dof but no order is required since the force simply applies to the equations related to the nodes, regardless of the order, with opposite magnitudes.

Output

The format is:

• the label of the element
• the label of the abstract node the force is applied to (the first node in case of abstract internal)
• the value of the force

Structural force

    <force_type> ::= { conservative | follower } 
    <force_arglist> ::= <node> , 
                        (Vec3) <relative_direction> ,
                        (Vec3) <relative_arm> ,
                        (drive_caller) <force_magnitude>

Structural internal force

    <force_type> ::= { conservative | follower } internal
    <force_arglist> ::= <node1> , 
                        (Vec3) <relative_direction> ,
                        (Vec3) <relative_arm1> ,
                        <node2> ,
                        (Vec3) <relative_arm2> ,
                        (drive_caller) <force_magnitude>

Output

The format is:

• the label of the element
• the label of the structural node the force is applied to
• the value of the force
• the three components of the force
• the arm of the force, in the global frame (i.e. referred to point $\left\{\vphantom{ 0, 0, 0 }\right.$0, 0, 0$\left.\vphantom{ 0, 0, 0 }\right\}$)

Example:

    # constant structural force
    force: 1, conservative,
        0.,0.,1., null,
        const, 100.;

Structural couple

    <force_type> ::= { conservative | follower } 
    <force_arglist> ::= <node> ,
                        (Vec3) <relative_direction> ,  
                        (drive_caller) <couple_magnitude>

Structural internal couple

    <force_type> ::= { conservative | follower } internal
    <force_arglist> ::= <node1> ,
                        (Vec3) <relative_direction> ,  
                        <node2> ,
                        (drive_caller) <couple_magnitude>

i.e. the arm is not required.

Output

The format is:

• the label of the element
• the label of the structural node the couple is applied to
• the value of the couple
• the three components of the couple

Note: by using a dof drive, a simple feedback control can be easily implemented.
A more general force element can be written by using a template drive that contains both the direction and the amplitude. This will be done in the future.