Contact US

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Multiple phase margins?

Multiple phase margins?

Multiple phase margins?

Hi all,

I am faced with an interesting transfer function as follows:

Gx(s) = C(s)*C_PID(s)*G(s)
(NB: "ex" represents 10^x)

C = 1/((5e-5)*s +1);
C_PID = Kp + Ki/s + Kd*s/(Tf*s+1) with Kp = 0.21205; Ki = 62.3191; Kd = 1.0773e-5; Tf = 2.9319e-5.
G =  (-342.1 s^3 - 3.713e10 s^2 + 5.446e15 s + 6.906e17)/(s^4 + 1288 s^3 + 3.733e10 s^2 + 6.275e12 s + 2.352e16)

Using the bode function in MATLAB shows that the open loop transfer function has multiple zero crossings. So, how does MATLAB decide which of the zero crossings should correspond to the phase margin? I ask because by tweaking C and C_PID slightly, similar plots are obtained but with a different zero crossing chosen (by MATLAB) for the phase margin calculation. Any help here?


Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members! Already a Member? Login


Close Box

Join Eng-Tips® Today!

Join your peers on the Internet's largest technical engineering professional community.
It's easy to join and it's free.

Here's Why Members Love Eng-Tips Forums:

Register now while it's still free!

Already a member? Close this window and log in.

Join Us             Close