how to solve the orror: Number of equations exceeds number of variables.

hello，I am trying to write a Simscape component that can define a fluid with variable density.The code is as follows:

component rho_fluid
    inputs
        density = { 1000, 'kg/m^3' }; % Fluid density, now an input parameter
    end 
    % Define parameters
    parameters
        viscosity_kin = { 18e-6, 'm^2/s' }; % Kinematic viscosity
        bulk = { 0.8e9, 'Pa' }; % Bulk modulus at atm. pressure and no gas
        alpha = { 0.005, '1' }; % Relative amount of trapped air
        range_error = { 2, '1' }; % Absolute pressure below absolute zero in blocks with fluid compressibility 
    end
 
    nodes
        G = foundation.hydraulic.hydraulic; % :right
    end
   
    equations
      
        assert(density > 0, 'Density must be positive');
        
        % Assign input and parameter values to the node
        G.density == density;  
        G.viscosity_kin == viscosity_kin;
        G.bulk == bulk;
        G.alpha == alpha;
        G.range_error == range_error; 
        assert(viscosity_kin > 0, 'Kinematic viscosity must be positive');
        assert(bulk > 0, 'Bulk modulus must be positive');
        assert(alpha >= 0, 'Alpha must be non-negative');
        assert(alpha < 1, 'Alpha must be less than 1');
    end
    end
    

I can succeesfully using 'ssc_build' to build the component，but when i run the model,the error occured:

Number of equations exceeds number of variables. Click on any Simscape blocks identified below for more detailed diagnostics. 

the picture is the simplified test model

Can anyone help me?matlab r2022b

[Umar]

Hi @ms z,

Upon reviewing the code, the primary issue lies in the use of the equality operator (==) instead of the assignment operator (=) when assigning values to the node properties. In Simscape, the correct operator for assigning values is =, while == is used for comparisons. This misuse leads to the equations not being properly defined, resulting in the error. So, to resolve the issue, replace the equality operator (==) with the assignment operator (=) in the equations section of the code. Here is the corrected code snippet:

equations
  assert(density > 0, 'Density must be positive');

    % Assign input and parameter values to the node
    G.density = density;  
    G.viscosity_kin = viscosity_kin;
    G.bulk = bulk;
    G.alpha = alpha;
    G.range_error = range_error; 
    assert(viscosity_kin > 0, 'Kinematic viscosity must be positive');
    assert(bulk > 0, 'Bulk modulus must be positive');
    assert(alpha >= 0, 'Alpha must be non-negative');
    assert(alpha < 1, 'Alpha must be less than 1');
  end

Hope this helps.

[Yifeng Tang]

Looks like this is a same question as another question you posted: https://www.mathworks.com/matlabcentral/answers/2159310-how-to-convert-constants-in-parameters-to-variables-in-inputs-in-simscape-invalid-use-of-value-dens?s_tid=srchtitle

I'll attempt to answer/comment there.

[Umar]

Hi @Yifeng Tang,

I did not know OP had a similar question posted in another post. Glad you caught it.

Answer 1

[Star Strider]

I do not have Simscape, so I looked at your Question simply out of interest.

Using Deploy a Component File in Block Diagram as a reference (there are others linked to it as well), it appears that you have not actually defined any variables in your posted code.