4569846498

Multibody systems simulation software 103
02
x
z
y
I
Action-only
force
x
z
y
J
03
Fig. 3.24
Action-only force
02
z
x
y
I
Torque acting
on the I marker
x
Reaction torque acting
on the J marker
z
y
J
03
Fig. 3.25
Action–reaction torque
Rotational forces may be defined to be action–reaction or action-only. In
either case the torque produced is assumed to act on the I marker. For an
action-only torque it is again the z-axis of the J marker that is used, in this
case to define the axis of rotation. If the torque is action–reaction the z-axes
of the I and J marker must be parallel and point in the same direction as
shown in Figure 3.25.
Considering next the definition of translational spring elements we can
start with a definition that is linear and introduce the use of system variables
for the formulation of a force. As can be seen in Figure 3.26 the formulation
of the spring force will be dependent on the length of the spring.
This is made available through a system variable defined here as DM(I, J)
which represents the scalar magnitude of the displacement between the I
and the J marker at any point in time during the simulation. The spring
force F S is initially defined here to be linear using
F S k(DM(I, J) L) (3.45)
where k is the spring stiffness and L is the free length of the spring, at zero
force.
The equation used in (3.45) to determine F S follows the required convention
that the scalar value of force produced is positive when the spring is in compression,
zero when it is at its free length and negative when it is in tension.