When I examine open loop transfer function L(s)= 50/(s^3+2s^2+s+4) using Routh-Hurowitz as well as the Roots[L(s)] function in Mathematica, both show two poles in right half plane. Yet, when I do a NyquistPlot[L(s) ], there is no encirclement of -1. Why is that?
Perhaps your syntax is incorrect. The documentation is a reliable friend.
ls = 50/(s^3 + 2 s^2 + s + 4)
tf = TransferFunctionModel[{{ls}}, s]
NyquistPlot[tf]
Solve[0 == 1/ls, s]
Hi Craig,
Thanks for the response. Attached is the code.
NyquistPlot.pdf (197 KB)
Oops! Never mind, my mistake. There should be zero encirclement’s since Z=N+P.=0+2=2.
You can explicitly see that the closed-loop system has 2 unstable poles
In[25]:= csys = SystemsModelFeedbackConnect@tf;
TransferFunctionPoles[csys][[1, 1]] // N
Out[26]= {-4.47411, 1.23705 - 3.2464 I, 1.23705 + 3.2464 I}
Or use the Routh–Hurwitz criterion which shows two sign changes in the first column.
In[27]:= Control`StabilityArray[csys][[1, 1]] // TableForm
