Problem with non linear fit model

I was trying to do the nonlinear fit model. Here is the function and program:

function Zel = TangentialElectrode1st(fre,Q4,a4,Rsep,Q2,a2,Rion,Rcont)
 Z1 = Rcont
 Zs = CPE (fre(:),Q2,a2)
 Z2 = ((Rion*Zs).^0.5).*coth((Rion./Zs).^0.5)
 Z3 = Rsep
 Z4 = CPE (fre(:),Q4,a4)

   Z23= (Z2*Z3)/(Z2+Z3)
   Z234 = (Z23*Z4)/(Z23+Z4)

   Zel = Z234+Z1
  end

modelfun = @(P,fre) TangentialElectrode1st(fre(:,1),P(1),P(2),P(3),P(4),P(5),P(6),P(7));
beta0 = [Q4,a4,Rsep,Q2,a2,Rion,Rcont]
mdl = nlinfit(fre,Zd,modelfun,beta0)

However, the matlab indicated that:

Error using nlinfit (line 211) MODELFUN must be a function that returns a vector of fitted values the same size as Y (150-by-1). The model function you provided returned a result that was 1-by-1.

One common reason for a size mismatch is using matrix operators (, /, ^) in your function instead of the corresponding elementwise operators (., ./, .^).

SO what could be the problem?

Answer 1

[Stephan]

Hi,

check the size of:

   Z23= (Z2*Z3)/(Z2+Z3)
   Z234 = (Z23*Z4)/(Z23+Z4)

maybe you should also use .* and ./ here

   Z23= (Z2.*Z3)./(Z2+Z3)
   Z234 = (Z23.*Z4)./(Z23+Z4)

Best regards

Stephan