weird closed loop step response for temperature control

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CONRAD CHAN
CONRAD CHAN 2019-7-15
评论： CONRAD CHAN 2019-7-20
I followed the Matlab code for the simulation of an open loop step response of a temperature control in the link below:
http://ctms.engin.umich.edu/CTMS/index.php?aux=Activities_Lightbulb
when I tried to do the close loop simulation for this system with my set point (T_set = 75), the rise time is strangely fast.
s = tf('s');
To = 18.5; % ambient/initial temperature
to = 0; % dead time response
K = 83.5; % DC gain
tau = 66; % time constant
T = K/(tau*s+1); % model transfer function
[y,t] = step(T,350); % model step response
% code for CL step response
T_set = 75;
opt2 = stepDataOptions('InputOffset',0,'StepAmplitude',T_set-To);
Kp = 1;
C = pid(Kp);
T_CL = feedback(T*C,1)
[y2,t2] = step(T_CL, opt2,350); % model step response
hold on
plot(t+to,y+To); %open loop step response
plot(t2+to,y2+To); % closed loop step response
xlabel('time (sec)')
ylabel('temperature (degrees C)')
legend('open loop','closed loop','Location','SouthEast')
hold off
I expected to get a simulation looks like this
However, my Closed Looped step response simulation does not look the similar to the above graph and the rise time is extremely fast.
would like to know what's the problem with the close loop transfer function?
Thank you

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