Support clocked variables?

[casella]

Since Modelica 3.3, clocked systems are defined in Modelica, see Chapter 16 of the language specification and this introductory paper.

Note that clocked variables are completely different from discrete variables. The former are only defined at events, while the latter change at events but are always defined. Also, the sampling of clocked systems is very rigourously defined through synchronous system theory, which is lacking for discrete variables.

In Modelica, clocks are propagated automatically according to synchronous semantics, which allows to determine synchronous clock partitions in a given system. According to the current Base Modelica draft specification, clock partitioning will be performed by the exporting tool, so the Base Modelica output already indicates the clock partitions explicitly and there is no need to figure them out by the importing tool. This makes implementing them in Base Modelica much easier than in Modelica.

Q1: is something similar to clocked variables and synchronous semantics defined in MTK?
Q2: if so, how hard would it be to map Base Modelica synchronous models with clocked partitions onto MTK?

Maybe @JKRT can also comment on that.

[JKRT]

https://docs.sciml.ai/ModelingToolkit/dev/tutorials/SampledData/ Is close; it is currently marked as experimental.

In OMJL I use https://docs.sciml.ai/DiffEqCallbacks/stable/timed_callbacks/ to emulate the semantics (Of some clock constructs, I do not yet support all of them).

If the pure MTK interface for this is not 100% complete I believe that the callback interface in DifferentialEquations.jl can be used as a temporary solution (Requires slightly more manually tinkering though).

[ChrisRackauckas]

No, this should use clocked variables, not callbacks.

[jClugstor]

Is there a good example of a BaseModelica model that has clocked variables?

[AnHeuermann]

All of Modelica.Clocked.Examples.Elementary demonstrate the basic usage of Clocks.

Maybe start with the Clock Constructors from the Modelica specification. According to MCP 0031 differences.md#clock-partitions a Modelica model RealSample

model RealSample
  Real y(start = 0, fixed = true);
  Clock c = Clock(0.1) \"Rational interval clock Operator 16.2\";
equation
  when c then
    y = pre(y) + 1;
  end when;
end RealSample;

should look something like the following Base Modelica:

//! base 0.1.0
package 'RealSample'
  model 'RealSample'
    Real 'y'(fixed = true, start = 0.0);
  equation
  partition /* Beginning of base-partition */
    Clock 'c' = Clock(0.1) "Rational interval clock Operator 16.2";

  subpartition (clock = 'c')
  equation
      'y' = pre('y') + 1.0;
  end 'RealSample';
end 'RealSample';

I don't think OpenModelica generates the correct Base Modelica yet, see OpenModelica/OpenModelica#15148.

[AnHeuermann]

I think the Base Modelica design isn't settled on a final version for clocked variables yet. Maybe you want to wait for that before adding any support for it.

[casella]

Based on today's discussion in the working group, chances are synchronous features could even be dropped from Base Modelica 1.0. So, this ticket is really low priority at the moment.