How to extract an MSL model, modify the code, and use locally?

Question

I am interested to replace my own PID-regulator models with MSL/Blocks/Continuous/LimPID. The problem is that this model restricts limitations of output signals to be parameters and thus do not allow time-varying limits, which I need to have.

Studying the code I understand that the output limitation is created by a block MSL/Blocks/Nonlinear/Limiter and I just want to change this to the block VariableLimiter.

I can imagine that you need to ensure that changes of output-limitations vary in a time-scale slower than the regulator in order to not excite unwanted behaviour of the controller. Still here is a class of problems where it would be very useful to allow this limits to vary slowly.

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Thanks for the good input to my question and below a very simple example to refine my question. (The LimPID is more complicated and I come back to that).

Let us instead just modify the block Add to a local block in MyModel.

I copy the code from Modelica.Blocks.Math.Add and call it Addb in MyModel. Since here is a dependence of Interfaces.SI2SO I need to make an import before the extends-clause. This import I take from the ordinary general MSL package, instead of copying also that in to MyModel. Then I introduce a new parameter "bias" and modify the equation. The annotation may need some update as well but we do not bother with that now.

MyModel
   ...

   block Addb "Output the sum of the two inputs"
      import Modelica.Blocks.Interfaces;
      extends Interfaces.SI2SO;
      parameter Real k1=+1   "Gain of input signal 1";
      parameter Real k2=+1   "Gain of input signal 2";
      parameter Real bias=0  "Bias term";
   equation
      y = k1*u1 + k2*u2 + bias;
      annotation (...);
   end Addb;
MyModel;

This code seems to work.

My added new question is whether it is enough to look up "extends-clauses" and other references to MSL and make the proper imports since the code is now local, or here are more aspects to think of? The LimPID code is rather complex with procedures for initialization etc so I just wonder if here is more to do than just bring in a number of import-clauses?

Answer 1

The models in Modelica Standard Library (MSL) should only be seen as exemplary models, not covering all possible applications. MSL is write protected and it is not possible to replace the limiter block in LimPID (and add max/min input connectors). Also, it wouldn't work out if you shared your simulation model with others, expecting their MSL to work like your modified MSL.

Personally, I have my own libraries of components where MSL models are inadequate. For example, I have PID controllers with variable limits, manual/automatic functions and many more functions which are needed in my applications.

Often, I create a copy of an MSL model, place it in the same package in my own library and make the necessary modifications and additions, e.g. MyLibrary.Blocks.Continuous.PID.