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--'); mod0909_2;