model RelativeStates "Definition of relative state variables"
extends Rotational.Interfaces.PartialTwoFlanges;
parameter StateSelect stateSelect = StateSelect.prefer "Priority to use the relative angle and relative speed as states";
parameter SI.Angle phi_nominal(displayUnit = "rad", min = 0) = 1 "Nominal value of the relative angle (used for scaling)";
SI.Angle phi_rel(start = 0, stateSelect = stateSelect, nominal = if Modelica.Constants.eps <= phi_nominal then phi_nominal else 1) "Relative rotation angle used as state variable";
SI.AngularVelocity w_rel(start = 0, stateSelect = stateSelect) "Relative angular velocity used as state variable";
SI.AngularAcceleration a_rel(start = 0) "Relative angular acceleration";
equation
a_rel = der(w_rel);
phi_rel = flange_b.phi - flange_a.phi;
w_rel = der(phi_rel);
flange_a.tau = 0;
flange_b.tau = 0;
annotation (
Documentation(info = "<html>\n<p>\nUsually, the absolute angle and the absolute angular velocity of\nModelica.Mechanics.Rotational.Components.Inertia models are used as state variables.\nIn some circumstances, relative quantities are better suited, e.g.,\nbecause it may be easier to supply initial values.\nIn such cases, model <strong>RelativeStates</strong> allows the definition of state variables\nin the following way:\n</p>\n<ul>\n<li> Connect an instance of this model between two flange connectors.</li>\n<li> The <strong>relative rotation angle</strong> and the <strong>relative angular velocity</strong>\n between the two connectors are used as <strong>state variables</strong>.</li>\n</ul>\n<p>\nAn example is given in the next figure\n</p>\n\n<p>\n<img src=\"modelica://Modelica/Resources/Images/Mechanics/Rotational/relativeStates.png\" alt=\"relativeStates\">\n</p>\n\n<p>\nHere, the relative angle and the relative angular velocity between\nthe two inertias are used as state variables. Additionally, the\nsimulator selects either the absolute angle and absolute angular\nvelocity of model inertia1 or of model inertia2 as state variables.\n</p>\n\n<p>\nSee also the discussion\n<a href=\"modelica://Modelica.Mechanics.Rotational.UsersGuide.StateSelection\">State Selection</a>\nin the User's Guide of the Rotational library.\n</p>\n</html>"),
Icon(
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end RelativeStates;