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POModelica - Modeling a slider element in OpenModelica
 

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    <p>Rheological models are usually build using three (or four) basics elements, which are :</p> <ul> <li><p>The <strong>spring</strong> (existing in <code>Modelica.Mechanics.Translational.Components</code> for example). Its equation is <code>f = c * (s_rel - s_rel0);</code></p></li> <li><p>The <strong>damper</strong> (dashpot) (also existing in <code>Modelica.Mechanics.Translational.Components</code>). Its equation is <code>f = d * v_rel;</code> for a linear damper, an could be easily modified to model a non-linear damper : <code>f = d * v_rel^(1/n);</code></p></li> <li><p>The <strong>slider</strong>, not existing (as far as I know) in this library... It's equation is abs(f)&lt;= flim. Unfortunately, I don't really understand how I could write the corresponding Modelica model...</p></li> </ul> <p>I think this model should extend <code>Modelica.Mechanics.Translational.Interfaces.PartialCompliant</code>, but the problem is that <code>f</code> (the force measured between <code>flange_b</code> and <code>flange_a</code>) should be modified only when it's greater than <code>flim</code>... If the slider extends <code>PartialCompliant</code>, it means that it already follows the equations <code>flange_b.f = f;</code> and <code>flange_a.f = -f;</code> Adding the equation <code>f = if abs(f)&gt;flim then sign(f)*flim else f;</code> gives me an error <i>"An independent subset of the model has imbalanced number of equations and variables"</i>, which I couldn't really explain, even if I understand that if <code>abs(f)&lt;=flim</code>, the equation <code>f = f</code> is useless... </p> <p>Actually, the slider element doesn't generate a new force (just like the spring does, depending on its strain, or just like the damper does, depending on its strain rate). The force is an input for the slider element, which is sometime modified (when this force becomes greater than the limit allowed by the element). That's why I don't really understand if I should define this force as an input or an output....</p> <p>If you have any suggestion, I would greatly appreciate it ! Thanks</p> <p>After the first two comments, I decided to add a picture that, I hope, will help you to understand the behaviour I'm trying to model. <img src="https://i.stack.imgur.com/DGsFE.png" alt="Different rheological elements"> On the left, you can see the four elements used to develop rheological models :</p> <ul> <li>a : the spring</li> <li>b : the linear damper (dashpot)</li> <li>c : the non-linear damper</li> <li>d : the slider</li> </ul> <p>On the right, you can see the behaviour I'm trying to reproduce : a and b are two associations with springs and c and d are respectively the expected stress / strain curves. I'm trying to model the same behaviour, except that I'm thinking in terms of force and not stress. As i said in the comment to Marco's answer, the curve a reminds me the behaviour of a diode :</p> <ul> <li>if the force applied to the component is less than the sliding limit, there is no relative displacement between the two flanges</li> <li>if the force becomes greater than the sliding limit, the force transmitted by the system equals the limit and there is relative displacement between flanges</li> </ul>
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