How can I work on simulink with symbolic representation variable ?

I want to create a transfer function using block diagram in simulink, e.g [1/(J*s+k)] . but when I linearise the system, Matlab is giving me the following error :

Invalid setting in 'untitled/Subsystem/Subsystem/Transfer Fcn' for parameter 'Denominator'.
Component:Simulink Control Design | Category:Linearization error
Error evaluating parameter 'Denominator' in 'untitled/Subsystem/Subsystem/Transfer Fcn'
Component:Simulink Control Design | Category:Linearization error
Undefined function or variable 'J'.
Component:Simulink Control Design | Category:Linearization error

Answer 1

[Walter Roberson]

The only part of MATLAB that supports symbolic variables in transfer functions is the Symbolic Toolbox -- and for that purpose "supports" just means that the symbolic toolbox has laplace transforms and fourier transforms, not that it has specific routines for control systems.

Within Simulink the closest you can get is tunable blocks. Those are transfer function representations in which you can change the value of a parameter, but that at any given time the parameter has a definite value.

[Anshuman S]

Does that mean I can't make a closed loop transfer function diagram with symbolic variables in order to calculate the systems open loop transfer function in symbolic variables ?

[Walter Roberson]

It means you cannot do it in Simulink, or using the Control Systems Toolbox in MATLAB. It might not be all that convenient using the Symbolic Toolbox, especially operations such as "linearise"

Now, if what you wanted was to create a transfer function with a parameter and you wanted Simulink to respond dynamically as you altered that parameter, with the parameter always having a defined "current" value, then there are some nice facilities for that, including facilities for tuning the parameter to get particular outputs.