Where are the other equations in my simscape block?

I have created my own simple custom simscape block. It should increase fluid temperature by X degrees. The block is build without any issues, but when I use it in a model, I get an error.

"Error compiling Simscape network for model untitled.
Caused by:
Number of equations exceeds number of variables. Click on any Simscape blocks identified below for more detailed diagnostics.
untitled/Increaser of fluid temperature (°C) (TL)"

The matlab window says:

Simscape diagnostics for block untitled/Increaser of fluid temperature (°C) (TL):
In the Simscape component Comp2.TemperatureIncrease, the number of equations exceeds the
number of continuous variables. The component only defines 4 variables, but 5 equations.
Possible causes are given below:

Caused by:
    Node TemperatureIncrease.B is possibly missing a branch statement for domain variable
    name: Phi.
    Node TemperatureIncrease.B is possibly missing a branch statement for domain variable
    name: mdot.
    Node TemperatureIncrease.A is possibly missing a branch statement for domain variable
    name: Phi.
    Node TemperatureIncrease.A is possibly missing a branch statement for domain variable
    name: mdot.
    The set of variables or outputs ( TemperatureIncrease.A.T, TemperatureIncrease.B.T ) is
    overdetermined because they are used in the following equations:
        In TemperatureIncrease (Line 21)
        An additional 2 automatically generated conservation equations.

My block code is:

component TemperatureIncrease
% Increaser of fluid temperature (°C) (TL)
% This block increases the temperature of the fluid by a user determined
% value

nodes
    A = foundation.thermal_liquid.thermal_liquid; % A:left
    B = foundation.thermal_liquid.thermal_liquid; % B:right
end

parameters
    dT = {10, 'K'}; % Temperature increase
end

% outputs
%     % New temperature of port B
%     T = {0, 'K'}; % T:right
% end

equations
    B.T == A.T + dT
    %T == A.T - B.T
end

end

What am I doing wrong?

Answer 1

[Sebastian Castro]

As your error message states:

    Node TemperatureIncrease.B is possibly missing a branch statement for domain variable
    name: Phi.
    Node TemperatureIncrease.B is possibly missing a branch statement for domain variable
    name: mdot.
    Node TemperatureIncrease.A is possibly missing a branch statement for domain variable
    name: Phi.
    Node TemperatureIncrease.A is possibly missing a branch statement for domain variable
    name: mdot.

In other words, you are missing a branches section defining the sign convention of the heat flux and mass flow rate at each port. If you look at the source code of some existing components (e.g. foundation.thermal_liquid.two_port_steady), you will see this section is included in the .ssc file.

branches
    mdot_A : A.mdot -> *;
    mdot_B : B.mdot -> *;
    Phi_A  : A.Phi  -> *;
    Phi_B  : B.Phi  -> *;
end

However, you'll see that a lot more may be missing from your code. I'd recommend using the inheritance feature in the Simscape Language by declaring your components as follows:

component TemperatureIncrease < foundation.thermal_liquid.two_port_steady

This will make sure all the necessary nodes/equations/variables are already defined, and you just fill in the rest of this "template" -- in your case, just the parameters and equations.

- Sebastian

[Jan Kragbæk]

Hi Sebastian Thank you very much for your answer. I'm having quite a hard time on these components. As you suggested, I tried to use the inheritance feature, but I still get errors. Do you know if there is anywhere, I can read more in details about thermal_liquid components language? Most examples I can find is about mechanical or electrical. If I use the inheritance code, how would you suggest the code whould look like, if I would like to simulate a temperature change and a pressureloss (dP = k*L/D*rho/2*V^2)?

- Jan

[Sebastian Castro]

You're right that the documentation is a little skewed towards the "easier" domains. There is some more information about the Thermal Liquid domain (and others) in the guide at http://www.mathworks.com/help/pdf_doc/physmod/simscape/simscape_lang.pdf

Besides that, I find the best success looking at the source code of existing components. If you look at the Simscape Foundation Library block dialogs, they should all have a "Source Code" hyperlink.

If you inherit from the foundation.thermal_liquid.two_port_steady component, all you need to define are any additional inputs/outputs/parameters in the block, as well as equations that relate the "through" (heat flux/mass flow) and "across" (pressure/temperature) variables.

- Sebastian

[Jan Kragbæk]

Hi Sebastian Since last, I have success with calculating the pressure loss with two.port.steady inherited. But adding equations for a temperature increase fails.

    dT  = {5, 'K'}; % Temperature increase
    T_B == T_A + dT

With dT under "Parameters" and T_B under "Equations, in". When running my code with this, I get that my number of equations exceed number of variables. And it says that my mass balance equation is the problem. What do I do?