Process Control: Modeling, Design, and Simulation

7.7. MATLAB Control Toolbox: Bode and Nyquist Functions

The MATLAB Control Toolbox has special functions to generate Bode and Nyquist plots, and to calculate gain and phase margins. Here we use Example 7.3 to illustrate the use of the MATLAB bode, nyquist, and imargin functions that are available in the Control Toolbox. For information, simply enter help bode, and so forth, in the MATLAB command window.

First, define the process transfer function (g_p).

» vdvtf = tf(0.5848*[-0.3549 1],[0.1858 0.8627 1])

Transfer function:
   -0.2075 s + 0.5848
-------------------------
0.1858 s^2 + 0.8627 s + 1

Next, generate the open-loop transfer function (g_cg_p) for a controller gain (k_c) of 2.5.

 » kc = 2.5; » gcgp = kc*vdvtf Transfer function: -0.5189 s + 1.462 ...