MATLAB Nyquist Plot Help: Transforming Numerator to Transfer Function"

[khedira]

i understand that the "nyquist" command in MATLAB requires one to key in the transfer function..

however, i am stuck here as i am unable to form a transfer function from the following numerator and denominator:

num = s-3/2-1/2*(-24*s+1)^(1/2)
den = 1.25*s^2 +3.75*s + 2.5

i know that the den can easily be form by [1.25 3.75 2.5] (assuming that numerator has degree of 2 only). however, how can i transform my num to be a tf form? please advise. thank you.

 

[MATLABdude]

According to the MATLAB Controls Toolbox documentation, you can form a transfer function from numerator and denominator using the tf command:
http://www.mathworks.com/help/toolbox/control/ref/tf.html

You can then run the nyquist function on the resultant transfer function:
http://www.mathworks.com/help/toolbox/control/ref/nyquist.html

 

[khedira]

According to the MATLAB Controls Toolbox documentation, you can form a transfer function from numerator and denominator using the tf command:
http://www.mathworks.com/help/toolbox/control/ref/tf.html

You can then run the nyquist function on the resultant transfer function:
http://www.mathworks.com/help/toolbox/control/ref/nyquist.html

Hi MATLABdude,

That is the problem. I have tried that before but it generates the following error:

"? Cell contents reference from a non-cell array object."

Do you have any resolution to the above error?

 

[MATLABdude]

Re-visit the MATLAB documentation page for Transfer Function:
http://www.mathworks.com/help/toolbox/control/ref/tf.html

Numerator and denominator are in the form of polynomial coefficients, not the actual symbolic form. For instance, s^2+2s-1 would be [1, 2, -1]. I believe this is the case with most of the controls toolbox.

EDIT: How did I miss the point of your post? Shows me to answer right before going to bed... Unfortunately, I'm not familiar with radical transfer functions. All of my familiarity is with purely rational transfer functions.

 

[DeeNos]

As a scientist, my advice would be to first simplify the numerator by using the quadratic formula to solve for the roots of the expression inside the square root. This will give you two distinct values for s, which you can then use to form a transfer function in the form of (s - a)(s - b). From there, you can use the MATLAB function "tf" to create the transfer function and then plot the Nyquist plot using the "nyquist" command. It is important to note that both the numerator and denominator must be in the form of a transfer function for the "nyquist" command to work properly. I would also recommend checking the MATLAB documentation or seeking help from a MATLAB expert for further assistance with this specific problem.