symbolic calculation of filter transmission function

Hello All,

here is my three-liner:

# #

clear all

syms s kp ki inp Hs out sum

[out]=solve('out=Hs/s','sum=inp-out','Hs=(kp+ki/s)*sum')

# #

I need an expression out(inp) .

The 3 equations are solvable, I did it by hand.

But matlab R2009b complains:

## Warning: Explicit solution could not be found. ##

Any hints, please?

Answer 1

[Martin Maschmann]

R2008a does the job

S=solve('out=Hs/s','sum=inp-out','Hs=(kp+ki/s)*sum','out,Hs,sum')

S =

     Hs: [1x1 sym]
    out: [1x1 sym]
    sum: [1x1 sym]

>> S.out

ans =

(kp*s+ki)*inp/(s^2+kp*s+ki)

>> S.out/inp ??? Undefined function or variable 'inp'.

>> syms inp >> S.out/inp

ans =

(kp*s+ki)/(s^2+kp*s+ki) -> this is what I want

[Walter Roberson]

In this version you specify the variables explicitly, but in the first one you did not. The three variables that would have been chosen to solve for are probably not the ones to choose to solve for.