L = 1/(s^3+1.5*s^2+1.5*s+1); % example of open-loop system
roots(L.den{:}) % confirm the open-loop system is stable 

figure(2);
nyquist(L, 'b'); % check if the unity feedback system is stable or not
hold on;

% 1st order system for perturbatoin
K = 1; % DC gain; stable
%K = 1.248; % DC gain; marginal stable
%K = 2; % DC gain; unstable

T = 0.001; % time constant(s); stable
%T = 0.2; % time constant(s); marginal stable
%T = 0.3; % time constant(s); unstable

Delta_L = K/(T*s+1);

nyquist(L*Delta_L, 'r--');

mod0914_2;
