如何定义参数之间的关系并保持灵活使用？

Question

如何定义多个参数之间的关系，以便我可以从集合中选择要定义的参数，一些方程式定义其余参数？

例如，我有一个包含 3 个参数的模型，分别是圆柱体的半径、高度和体积，它们通过体积方程 V=r^2*h 相关联。如果我定义 r 和 h（并且计算 V），我希望能够选择模型实例化，r，V（和 h 被计算），或 h、V（并且 r 被计算）。

在下面的最小示例中，我尝试了方法 1-3，但 none 满足了我的需要。我感觉这一定是一个 common/solved 问题，我只是缺少某种建模技术。你能帮忙吗？

model test_params "Model for a cylinder"
  model Cylinder
  parameter Real r;
  parameter Real h;
  parameter Real V;
  //Approach 3: binding equation. Works, but what to do if I have V, h and want to know r??
  //parameter Real V=r^2*Modelica.Constants.pi*h;

  Real t;
  initial equation
    //Approach 2, apparently chooses V as 0 before getting to the initialisation equation
    //V=r^2*Modelica.Constants.pi*h; 
    //Parameter cylinder.V has no value, and is fixed during initialization (fixed=true), using available start value (start=0.0) as default value.
    // The initial conditions are over specified. The following 1 initial equations are redundant, so they are removed from the initialization sytem:
    //    cylinder.V = 3.141592653589793 * cylinder.r ^ 2.0 * cylinder.h.
  equation
    t = V;
    // Approach 1
    //V=r^2*Modelica.Constants.pi*h; //Leads to: Too many equations, over-determined system.
  end Cylinder;

  test_params.Cylinder cylinder(h = 1, r = 2)  annotation(
    Placement(visible = true, transformation(origin = {-44, 30}, extent = {{-10, -10}, {10, 10}}, rotation = 0)));
equation
end test_params;

这是在 OpenModelica 1.14 上，如果相关的话。

Answer 1

如果您将依赖参数设置为 fixed=false 参数，则可以使用您提到的方法 2 来完成此操作。

model TestParams "Model for a cylinder"
  encapsulated model Cylinder
    import Modelica.Constants.pi;
    parameter Real r;
    parameter Real h;
    parameter Real V;
  initial equation 
    V = r^2*pi*h "Cylinder volume";
  end Cylinder;

  TestParams.Cylinder cylinder_1(h=1, r=2, V(fixed=false)) "Compute cylinder V(h, r)"
    annotation (Placement(transformation(origin={-70,50}, extent={{-10,-10},{10,10}})));
  TestParams.Cylinder cylinder_2(h(start=0, fixed=false), r=2, V=13) "Compute cylinder h(r, V)"
    annotation (Placement(transformation(origin={-30,50}, extent={{-10,-10},{10,10}})));
  TestParams.Cylinder cylinder_3(h=1, r(start=0, fixed=false), V=13) "Compute cylinder r(h, V)"
    annotation (Placement(transformation(origin={10,50}, extent={{-10,-10},{10,10}})));
end TestParams;

Answer 2

我建议，为了有一个图形选择，类似于：

package test_params2 "Model for a cylinder"

type InputChoice = enumeration(rh,rv,hv);

model Cylinder
  parameter InputChoice choice;
  parameter Real r;
  parameter Real h;
  parameter Real v;
  Real R,H,V;
equation
  if choice==InputChoice.rh then
    R=r;
    H=h;
  elseif choice==InputChoice.rv then
    R=r;
    V=v;
  else
    H=h;
    V=v;
  end if;
  V=R^2*Modelica.Constants.pi*H;
end Cylinder;

model test_params
  Cylinder cylinder(choice = test_params2.InputChoice.rh,h=1,r=2, v = 30)       annotation(
Placement(visible = true, transformation(origin = {-44, 30}, extent = {{-10, -10}, {10, 10}}, rotation = 0)));
end test_params;

end test_params2;

如何定义参数之间的关系并保持灵活使用？

How do I define relations between parameters and retain flexible use?

modelica

openmodelica