.Modelica.Mechanics.Rotational.UsersGuide.SignConventions .Modelica.Mechanics.Rotational.UsersGuide.SignConventionsThe variables of a component of this library can be accessed in the usual way. However, since most of these variables are basically elements of vectors, i.e., have a direction, the question arises how the signs of variables shall be interpreted. The basic idea is explained at hand of the following figure:
In the figure, three identical drive trains are shown. The only
difference is that the gear of the middle drive train and the gear
as well as the right inertia of the lower drive train are
horizontally flipped with regards to the upper drive train. The
signs of variables are now interpreted in the following way: Due to
the 1-dimensional nature of the model, all components are basically
connected together along one line (more complicated cases are
discussed below). First, one has to define a
positive direction of this line, called
axis of rotation. In the top part of the figure
this is characterized by an arrow defined as axis of
rotation. The simple rule is now: If a variable of a
component is positive and can be interpreted as the element of a
vector (e.g., torque or angular velocity vector), the corresponding
vector is directed into the positive direction of the axis of
rotation. In the following figure, the right-most inertias of the
figure above are displayed with the positive vector direction
displayed according to this rule:
The cut-torques J2.flange_a.tau, J4.flange_a.tau,
J6.flange_b.tau of the right inertias are all identical and
are directed into the direction of rotation if the values are
positive. Similarly, the angular velocities J2.w, J4.w,
J6.w of the right inertias are all identical and are also
directed into the direction of rotation if the values are positive.
Some special cases are shown in the next figure:
In the upper part of the figure, two variants of the connection
of an external torque and an inertia are shown. In both cases, a
positive signal input into the torque component accelerates the
inertias inertia1, inertia2 into the positive axis of
rotation, i.e., the angular accelerations inertia1.a,
inertia2.a are positive and are directed along the "axis of
rotation" arrow. In the lower part of the figure the connection of
inertias with a planetary gear is shown. Note, that the three
flanges of the planetary gearbox are located along the axis of
rotation and that the axis direction determines the positive
rotation along these flanges. As a result, the positive rotation
for inertia4, inertia6 is as indicated with the
additional grey arrows.