4569846498

Multibody systems simulation software 105
I
I
As markers
approach VR(I,J )
is negative
As markers
separate VR(I,J )
is positive
J
J
Fig. 3.28
Damper force is positive
Damper force is negative
Sign convention for damper forces and velocities
An alternative form of definition that could be used with MSC.ADAMS
involves the use of the SPRINGDAMPER statement. In this case this
would have exactly the same effect as the SFORCE statement above:
SPRING/0509,I0205,J0409,TRANS,K40,L250
In a similar manner to the definition of a spring force the representation of
a damper force will involve using the line-of-sight method to formulate an
action–reaction force between an I marker on one part and a J marker on
another part. We will again start with the linear case where we formulate a
damper force F D using
F D c • VR(I, J) (3.46)
where
VR(I, J) radial line of sight velocity between I and J marker
c damping coefficient
Since the force generated in a damper is related to the sliding velocity acting
along the axis of the damper we introduce another system variable VR(I, J)
that will take a positive sign when the markers are separating, as in suspension
rebound, and a negative sign when the markers are approaching, as
when a suspension moves upwards relative to the body in bump. The formulation
of the damper force F D in (3.46) is such that the damper forces are
consistent with those of a spring. The force generated is positive in a repelling
mode and negative in an attracting mode as illustrated in Figure 3.28.
The formulation of the damper force F D is shown graphically in Figure 3.29.
An example of the syntax that could be used to formulate this in
MSC.ADAMS, using an SFORCE statement, would be
SFORCE/0509,I0205,J0409,TRANS,FUNCTION
-5*VR(0205,0409)