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Step response of a closed-loop system

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Alexander Babin
Alexander Babin on 6 Feb 2020 at 6:57
Edited: Alexander Babin on 6 Feb 2020 at 6:59
Dear friends,
i need to determine the stability of a system that consists of a PI controller, a servovalve and a thrust bearing with its reaction force linearised. all transfer functions are in the model attached to the question. if i model the step response to the change of the set point, it works just fine, however derivation of a TF for the system in the following way:
W1=tf([1000 5000],[1 0]); % PI transfer function
W2_02=tf([0 250],[0.02 1]); % Servovalve transfer function with time constant 0.02 s
W2_12=tf([0 250],[0.12 1]); % Servovalve transfer function with time constant 0.12 s
W3=tf([1],[7.5 4e4 5.5e5]); % bearing transfer function
Wsys_02=W1*W2_02*W3; % open loop transfer functions
Wsys_12=W1*W2_12*W3;
CL_02=tf([Wsys_02],[1+Wsys_02]); % closed loop transfer functions
CL_12=tf([Wsys_12],[1+Wsys_12]);
results in the command step(CL_02) to show an infinitely increasing response.
There must be something I am missing, please, if anyone has any ideas, do share with me! Thanks a lot!

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