I quote an example of *Closing Feedback Loops with Time Delays*at http://www.mathworks.cn/help/toolbox/control/ug/bstzkhr.html#bstz1u7 I get prompt like this [red] ??? Undefined command/function 'pid'.[/red] and when I change to tf form, I get ??? Error using ==> tf.feedback FEEDBACK cannot handle time delays. What's matter? How to correct? G = tf(1,[1 10],'InputDelay',2.1) C= tf([0.5 2.3], [1 0]) %C = pid(0.5,2.3); % C has a proportional gain of 0.5 and an integral gain of 2.3. % Use feedback to compute the closed-loop response from r to y. T = feedback(C*G,1); Transfer function: 1 exp(-2.1*s) * ------ s + 10 Transfer function: 0.5 s + 2.3 ----------- s ??? Error using ==> tf.feedback FEEDBACK cannot handle time delays.

G = tf(1,[1 10],'InputDelay',2.1) 
C= tf([0.5 2.3], [1 0])
%C = pid(0.5,2.3);
% C has a proportional gain of 0.5 and an integral gain of 2.3.
% Use feedback to compute the closed-loop response from r to y.
T = feedback(C*G,1);
Transfer function:
                1
exp(-2.1*s) * ------
              s + 10
Transfer function:
0.5 s + 2.3
-----------
     s
??? Error using ==> tf.feedback
FEEDBACK cannot handle time delays.