Syntax Error using Fcn Block

I have been trying to use Fcn Block in simulink but i get the error

The expression: (d1*h11)*(u(7))-(d1*h12+d1*C1)*(u(2))+(d2*h7+d2*h8+d3*h10)*(u(3))-(d2*C2+d3*C3)*(u(4))-d3*C3*(u(6))+d3*h10*(u(5))

in 'slx_drips/DRIPS/Fcn1'

has a syntax error

Please can somebody solve this error.

Thanks

Answer 1

[sanidhyak]

Hi @Tufail Ahmad Bhat,

I understand that you are encountering a syntax error in the “Fcn” block of Simulink when trying to use an expression that includes variables such as “d1”, “h11”, “C1”, etc. This issue arises because Simulink’s “Fcn” block does not support named variables—it only permits operations involving constants and the input vector “u(i)”.

To resolve this issue, I recommend either precomputing the constants or switching to a “MATLAB Function" block, which supports named variables. Please refer to the following approach using a “MATLAB Function" block:

function y = customFcn(u) 
% Define constants 
d1 = ...; h11 = ...; h12 = ...; C1 = ...; 
    d2 = ...; h7 = ...; h8 = ...; C2 = ...; 
    d3 = ...; h10 = ...; C3 = ...; 
    
% Compute expression 
y = (d1*h11)*u(7) - (d1*h12 + d1*C1)*u(2) + ... 
    (d2*h7 + d2*h8 + d3*h10)*u(3) - ... 
    (d2*C2 + d3*C3)*u(4) - d3*C3*u(6) + d3*h10*u(5); 
end 

You can insert this block by navigating to “Simulink Library > User-Defined Functions > MATLAB Function” and double-clicking it to edit the code.

Alternatively, if you still wish to use the “Fcn" block, you must precompute all variables like “d1*h11”, etc., externally (in MATLAB workspace or Constant blocks), and only use those scalar values in the block.

For more information on how the “Fcn” block works, you may refer to the official documentation:

• https://www.mathworks.com/help/simulink/slref/fcn.html

I hope this helps!