5 ms·

Modelica

57 points by Immortalin 9y ago

dmlorenzetti 9y ago
Modelica sees a lot of use in building energy simulation [0], [1]. My impression is that the solver landscape is a little uneven (the solver reads a Modelica model and computes its solution). For one thing, open-source solvers seem to lag behind the commercial implementations, in terms of capabilities. Even with commercial solvers, there seems to be a non-negligible amount of work around taking correct models that don't run, and reformulating them to let the solvers run them. Caveats -- I don't work on this personally, but know people in my division who do. Also, these impressions are a few years old, and doubtless the situation is improving as time goes on. [0] Library of component modules for building energy and control systems (http://simulationresearch.lbl.gov/modelica/ http://simulationresearch.lbl.gov/modelica/) [1] "Spawn of EnergyPlus", a prototype next-gen whole-building energy solver (https://energy.gov/eere/buildings/downloads/spawn-energyplus-soep https://energy.gov/eere/buildings/downloads/spawn-energyplus...)
- mcguire 9y ago
  It's a fascinating example of "near-programming", computing results declaratively, if I'm understanding the wiki. I'm actually surprised it works at all.
  - lgas 9y ago
    > I'm actually surprised it works at all. Why?
    - mcguire 9y ago
      I wouldn't expect it to find a stable solution.
  - tkuraku 9y ago
    I have used Modelica for some classes. It works really well. You can simulate continuous and discrete time systems all in the same model. It it has some similarities to Mathworks' Simulink and Wolfram SystemModeler actually uses the Modelica language.
nraynaud 9y ago
I have a friend who is or used to help define this thing, it's incredibly fiddly: mixed domain/mixed energy/correct units*correct representation types, and has to produce a result.
- MechEStudent 9y ago
  It looked like a way to make a closed ecosystem where there were only a few entrants (users) and the cost of entry was high both economically and in terms of the learning curve. It is nice to see that I'm not entirely wrong.
kxyvr 9y ago
I've been interested in Modelica for some time and didn't know if someone could answer a few questions. I'm interested in building what I call nodal models, but basically these are things like multiphysics models similar to circuits. Essentially, this is the bond graph methodology, but I don't think the bond graph notation is strictly necessary, so really it's just hooking things like electrical circuit models with thermal or mechanical components. Anyway, my questions are: 1. Does anyone know the history of these software tools? From what I've been able to gather, before Modelica people used ACSL (https://en.wikipedia.org/wiki/Advanced_Continuous_Simulation_Language https://en.wikipedia.org/wiki/Advanced_Continuous_Simulation...), but currently products like Simulink or Amesim are more common. 2. I thought that the modern packages moved from traditional integrators like Runge-Kutta methods to QSS methods. In the applied math world/ODE/PDE world, I've never seen anyone use a QSS method. Are QSS methods general and how do they compare practically to more traditional explicit time integrators? Anyway I'm glad to see Modelica get posted and thanks in advance for answering the questions above.
- rhodin 9y ago
  1. Yes you can say that Modelica has some heritage from ACSL. (See https://www.modelica.org/publications/papers/esm98his.pdf https://www.modelica.org/publications/papers/esm98his.pdf) but the short story is that some simulation language creators decided to form a uniform modeling and simulation language together instead of working on their own initiatives, resulting in Modelica. 2. Not really. Usually modified versions of DASSL/CVODES and friends are used by Modelica tools. In Cellier and Kofmans's book on simulation QSS gets prominent display and some of the research at Berkley Labs has been using QSS to some extent, but it is far from mainstream.
- dmlorenzetti 9y ago
  If you're interested in circuit-like systems, Modelica is probably a good fit. Electrical networks seem to be a core use case for the developers/presenters of Modelica. Also I know it has been used to model airflow networks in buildings, using the multizone approach which typically uses a nodal formulation. If you've read much about Modelica, you probably know this, but one key to thinking about Modelica is to recognize that you're not defining a nodal system anymore -- at least, not in the way that an electrical engineering textbook would define it. Yes, you can define a network, which can have resistive/capacitive/inductive elements strung between nodes. However, in EE-speak, you would then form either the nodal equations or the mesh equations, depending on whether you wanted to add up currents into nodes, or voltages around loops. But in Modelica, you just state the component relations, and the rules that apply to the connections between components. Then the solver decides how to set up the system equations. Typically the solver uses graph-theoretical techniques to reduce the number of unknowns, or to achieve other nice system properties. So for example if you had a bunch of components in series, you might expect the common current to be a state variable, whereas in another part of the system you might find voltage state variables. Modelica encourages you to think about the components, and let the solver handle the system assembly and solution. I believe Modelica got its start in the automotive simulation world, where there is a need for large-scale lumped-parameter multiphysics models. I'm not sure how ACSL fits in. In the building simulation world, there was also an earlier language called Neutral Model Format that attacked many of the same problems as Modelica. It's been a long time since I studied bond graphs, but I would say Modelica is much more general, since it allows discrete as well as continuous components, and since it doesn't require the formalism of identifying the equivalent of voltages and currents in each domain (don't remember what bond graph theory calls those). I think Modelica even allows components to be defined by algorithms, although obviously those components may not have mathematical inverses that can be easily manipulated when assembling the system.
analognoise 9y ago
For anyone interested, the OpenModelica compiler is available, and aims to be the standard reference implementation: https://openmodelica.org https://openmodelica.org

57 points by Immortalin 9y ago

Modelica sees a lot of use in building energy simulation [0], [1]. My impression is that the solver landscape is a little uneven (the solver reads a Modelica model and computes its solution). For one thing, open-source solvers seem to lag behind the commercial implementations, in terms of capabilities. Even with commercial solvers, there seems to be a non-negligible amount of work around taking correct models that don't run, and reformulating them to let the solvers run them. Caveats -- I don't work on this personally, but know people in my division who do. Also, these impressions are a few years old, and doubtless the situation is improving as time goes on. [0] Library of component modules for building energy and control systems (http://simulationresearch.lbl.gov/modelica/ http://simulationresearch.lbl.gov/modelica/) [1] "Spawn of EnergyPlus", a prototype next-gen whole-building energy solver (https://energy.gov/eere/buildings/downloads/spawn-energyplus-soep https://energy.gov/eere/buildings/downloads/spawn-energyplus...)

mcguire 9y ago
It's a fascinating example of "near-programming", computing results declaratively, if I'm understanding the wiki. I'm actually surprised it works at all.
- lgas 9y ago
  > I'm actually surprised it works at all. Why?
  - mcguire 9y ago
    I wouldn't expect it to find a stable solution.
- tkuraku 9y ago
  I have used Modelica for some classes. It works really well. You can simulate continuous and discrete time systems all in the same model. It it has some similarities to Mathworks' Simulink and Wolfram SystemModeler actually uses the Modelica language.

nraynaud 9y ago

I have a friend who is or used to help define this thing, it's incredibly fiddly: mixed domain/mixed energy/correct units*correct representation types, and has to produce a result.

MechEStudent 9y ago
It looked like a way to make a closed ecosystem where there were only a few entrants (users) and the cost of entry was high both economically and in terms of the learning curve. It is nice to see that I'm not entirely wrong.

kxyvr 9y ago

I've been interested in Modelica for some time and didn't know if someone could answer a few questions. I'm interested in building what I call nodal models, but basically these are things like multiphysics models similar to circuits. Essentially, this is the bond graph methodology, but I don't think the bond graph notation is strictly necessary, so really it's just hooking things like electrical circuit models with thermal or mechanical components. Anyway, my questions are: 1. Does anyone know the history of these software tools? From what I've been able to gather, before Modelica people used ACSL (https://en.wikipedia.org/wiki/Advanced_Continuous_Simulation_Language https://en.wikipedia.org/wiki/Advanced_Continuous_Simulation...), but currently products like Simulink or Amesim are more common. 2. I thought that the modern packages moved from traditional integrators like Runge-Kutta methods to QSS methods. In the applied math world/ODE/PDE world, I've never seen anyone use a QSS method. Are QSS methods general and how do they compare practically to more traditional explicit time integrators? Anyway I'm glad to see Modelica get posted and thanks in advance for answering the questions above.

rhodin 9y ago
1. Yes you can say that Modelica has some heritage from ACSL. (See https://www.modelica.org/publications/papers/esm98his.pdf https://www.modelica.org/publications/papers/esm98his.pdf) but the short story is that some simulation language creators decided to form a uniform modeling and simulation language together instead of working on their own initiatives, resulting in Modelica. 2. Not really. Usually modified versions of DASSL/CVODES and friends are used by Modelica tools. In Cellier and Kofmans's book on simulation QSS gets prominent display and some of the research at Berkley Labs has been using QSS to some extent, but it is far from mainstream.
dmlorenzetti 9y ago
If you're interested in circuit-like systems, Modelica is probably a good fit. Electrical networks seem to be a core use case for the developers/presenters of Modelica. Also I know it has been used to model airflow networks in buildings, using the multizone approach which typically uses a nodal formulation. If you've read much about Modelica, you probably know this, but one key to thinking about Modelica is to recognize that you're not defining a nodal system anymore -- at least, not in the way that an electrical engineering textbook would define it. Yes, you can define a network, which can have resistive/capacitive/inductive elements strung between nodes. However, in EE-speak, you would then form either the nodal equations or the mesh equations, depending on whether you wanted to add up currents into nodes, or voltages around loops. But in Modelica, you just state the component relations, and the rules that apply to the connections between components. Then the solver decides how to set up the system equations. Typically the solver uses graph-theoretical techniques to reduce the number of unknowns, or to achieve other nice system properties. So for example if you had a bunch of components in series, you might expect the common current to be a state variable, whereas in another part of the system you might find voltage state variables. Modelica encourages you to think about the components, and let the solver handle the system assembly and solution. I believe Modelica got its start in the automotive simulation world, where there is a need for large-scale lumped-parameter multiphysics models. I'm not sure how ACSL fits in. In the building simulation world, there was also an earlier language called Neutral Model Format that attacked many of the same problems as Modelica. It's been a long time since I studied bond graphs, but I would say Modelica is much more general, since it allows discrete as well as continuous components, and since it doesn't require the formalism of identifying the equivalent of voltages and currents in each domain (don't remember what bond graph theory calls those). I think Modelica even allows components to be defined by algorithms, although obviously those components may not have mathematical inverses that can be easily manipulated when assembling the system.

analognoise 9y ago

For anyone interested, the OpenModelica compiler is available, and aims to be the standard reference implementation: https://openmodelica.org https://openmodelica.org