how to resolve conflicts with calling the 'tf' function between different toolboxes?

I am having trouble calling the function 'tf' to define transfer function variables in the Control Systems Toolbox. The problem began when I recently upgraded Matlab to 2016a (the problem didn't happen with Matlab 2014). When I type 'ver' it shows that I have the Control Systems Toolbox installed, however when I try to call the function I get this error:

Undefined function 'tf' for input arguments of type 'char'.

To try to figure out version of the function is getting called, I typed:

>> which('tf') /Applications/MATLAB_R2016a.app/toolbox/dsp/filterdesign/@mfilt/tf.m % mfilt method

Does anyone know how to get Matlab to call the Control Systems version of the function, without deleting the Filter Design Toolbox?

Answer 1

[Star Strider]

You didn’t post your relevant code, so I’m not certain how you’re calling the function. When I looked at the documentation for the two tf functions (in the Signal Processing and Control System Toolboxes) in R2016a, neither of them takes a string or character argument.

[Jason]

Thanks for your fast reply to this post. I am trying to initialize a transfer function variable so that I can use it with Controls Systems Toolbox functions like 'feedback', 'margin', and 'step'. The code that causes the error is:

s = tf('s');

[Star Strider]

I’m not certain what the problem may be with your installation. When I do:

tfs = tf('s')

it returns:

tfs =

    s

Continuous-time transfer function.

My only other suggestion is to type this in the Command Window:

restoredefaultpath 
rehash toolboxcache

since a path problem is the only other possibility I can think of.

[Jason]

Thank you! These commands solved the problem. I'm not sure either what went wrong with the installation... this is my first time trying to use the Control Systems Toolbox since upgrading to 2016a last week.

[Star Strider]

My pleasure!

Such problems aren’t uncommon, and can occasionally appear out of nowhere. That’s the reason I keep that bit of code in my ‘Answers.m’ file that I use to test code I post here. I’ve also had to use it a couple times over the years.