Unit problem in a Simscape custom block

Hi everyone,

I'm facing a unit problem when building a Simscape custom block. I'm using the flow rate (Q) that I imported in the section 'branches' and declared as a variable specifying the unit (m^3/s). But when I use Q in the equation section it happened to be in cm^3/s which make my equation wrong. How can I fix that ?

Thanks in advance,

Nils

Answer 1

[Anushka]

Hi @Nils,

According to my understanding, the error suggests that there is a mismatch or incorrect assignment of units within the component, specifically related to the flow rate 'Q'.

Here are some steps and tips to resolve unit conversion issues in Simscape:

1. Global Unit Setting in Simulink Configuration:

• Go to the Simulink model’s Configuration Parameters window.
• Navigate to "Simscape" and check the default units and unit conversion settings to ensure that they align with your component expectations.
• Make changes if needed to ensure consistency in units like m^3/s for flow rate.

2. Check Variable Declarations:

• Verify that there are no other initializations or assignments within your code that might implicitly convert units. For example, confirm that 'Q' is declared without any units indicated like cm^3/s outside of your immediate code.

3. Custom Unit Definition (if needed):

• Create a custom unit within the Simulink model, if applicable, to specify m^3/s. This can force the system to use the desired dimension.

4. Reviewing Component Equations:

• In your component equations, ensure all quantities used have compatible units. Check divisions and multiplications, especially those involving constants and parameters (rho,g,r).

Once the changes are made:

• Rebuild the Model: After revising the unit settings and variable declarations, rebuild and simulate the model to check for errors.
• Unit Verification: Utilize Simscape logging to verify the units of all variables during simulation. This logging can help ensure that 'Q' is indeed using m^3/sas intended.

Hope this helps!

[Nils]

Hi @Anushka,

Thanks for your message. I've already changed the 'nominal values' in the Configuration Parameters window in my Simscape model (it was indeed in cm^3/s) and started the program again, but it didn't work. For the points 2 and 4 I think it is fine. About creating a custom unit definition, it suggest that the component is already created and that you can run the simulation no ? I send my code in case you want to review it.

component pelton_turbine

nodes

H = foundation.hydraulic.hydraulic;

R = foundation.mechanical.rotational.rotational;

end

parameters

eta = {0.9, '1'};

rho = {1000, 'kg/m^3'};

r = {0.5, 'm'};

g = {9.81, 'm/s^2'};

end

variables

Q = {0, 'm^3/s'};

T = {0, 'N*m'};

H_head = {0, 'm'};

end

branches

Q : H.q -> *;

end

equations

H_head == H.p/ (rho * g);

T == {eta * rho * Q * r * sqrt(H_head * g/2), 'N*m'};

R.t == T;

end

end

[Anushka]

Hello,

Upon reviewing your Simscape component, the issue arises due to the incorrect unit handling in the equations section. The line:

T == {eta * rho * Q * r * sqrt(H_head * g/2), 'N*m'}; 

is applying the unit to the entire expression, but Simscape already handles unit consistency automatically during simulation. The {} syntax is only needed for variable or parameter definitions, not in dynamic equations.

The following steps will help you resolve the issue:

1. Fix the Equation Syntax:

Replace:

T == {eta * rho * Q * r * sqrt(H_head * g/2), 'N*m'};

With:

  T == eta * rho * Q * r * sqrt(H_head * g/2);

This will ensure the units are handled correctly during simulation.

2. Force Unit Consistency (if needed):

If the issues persists, explicitly cast the units using:

 T == eta * rho * unitConversionFactor(Q, 'm^3/s') * r * sqrt(H_head * g/2);

This guarantees that Q is treated in m^3/s during computation.

After making these changes , rebuild and simulate your model. You can use Simscape logging to verify the units during the simulation and confirm that Q is in m^3/s as expected.

[Nils]

The first solution worked, thanks a lot for your help !