What does "Equation is not a function of any variables. Each equation must be a function of at least one variable." mean?

Hello,

I am trying to modify inductor element from Simscape library. I want to change the inductance depending on the current flowing through. I wrote the following piece of code:

component inductor < foundation.electrical.branch
  parameters
    l  = { 1e-6, 'H' };     % Inductance
    i0 = { 0, 'A' };        % Initial current
    r  = { 0, 'Ohm' };      % Series resistance
    g  = { 1e-9, '1/Ohm' }; % Parallel conductance
  end

variables
  i_L = { 0, 'A' }; % Internal variable for current through inductor term
end

function setup
  if l <= 0
      pm_error('simscape:GreaterThanZero','Inductance')
  end
  if g < 0
      pm_error('simscape:GreaterThanOrEqualToZero','Parallel conductance')
  end
  if g == {inf, '1/Ohm'}
      pm_error('simscape:LessThan','Parallel conductance','inf')
  end   
  if r < 0
      pm_error('simscape:GreaterThanOrEqualToZero','Series resistance')
  end
  if r == {inf, 'Ohm'}
      pm_error('simscape:LessThan','Series resistance','inf')
  end
  i_L = i0; % Assign initial current
end

equations
    if i_L > { 10, 'A'}
        l == 0.5 * l + {0, 'H'} * i_L / { 1, 'A' };
    else
        l == l + {0, 'H'} * i_L / { 1, 'A' };
    end

    v == i_L*r + l*i_L.der;
    i == v*g + i_L;
  end
end

And when I try to build a model, I get an error:

              >> ssc_    build electrical
              Generating 'electrical_lib.mdl' in the current directory 'c:\Michal\MATLAB Simulations\Thermal simulations' ...
              ??? Failed to generate 'electrical_lib'

              Caused by:
                  Error using ==> inductor>equations at 33
                  Equation is not a function of any variables. Each equation must be a function of at least one variable.

Could you please tell me what does it mean, and what should I do to implement this feature.

Thank you in advance for any help you can offer.

Best regards,

Michal

Answer 1

[Sabin]

The problem is in the if statement. In both branches you have '{0, 'H'} * i_L' which means that i_L is eliminated. As 'l' is a parameter it means there is no variable left in the equations.

In latest releases the code would build but will not simulate as the number of equations exceed number of variables.

If the goal is to modify the inductance probably best to use an intermediate, something like:

intermediates
    l_new = if (condition), ... else ... end;
end