How do I define the limits of a discrete model? Why do I receive this error for every t and sht variables?

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sadel 2011-6-3

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https://ww2.mathworks.cn/matlabcentral/answers/8748-how-do-i-define-the-limits-of-a-discrete-model-why-do-i-receive-this-error-for-every-t-and-sht-vari

   t= 0:0.01:10;
x=tf([1],[1 1]);
sht=0.5;
cd=c2d(x,sht,'zoh')

Transfer function: 0.3935 ---------- z - 0.6065

Sampling time (seconds): 0.5 step(cd,t)

??? Error using ==> DynamicSystem.step at 84 When specifying a time vector for time response simulations, the time step match the sampling time of discrete-time models.

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Fangjun Jiang 2011-6-3

Me too. Definitely need to avoid that.

sadel 2011-6-3

lol Are you talking about 'sht'? Sample Hold Time

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采纳的回答

Fangjun Jiang 2011-6-3

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https://ww2.mathworks.cn/matlabcentral/answers/8748-how-do-i-define-the-limits-of-a-discrete-model-why-do-i-receive-this-error-for-every-t-and-sht-vari#answer_12063

Your "t" indicates 0.01 as the sample time, but your c2d(x,sht,'zoh') indicate 0.5 as the sample time. You need to make them the same and it will work.

You are doing a continues transfer function to discrete transfer function conversion, which is sample time dependent. It doesn't make sense to run your discrete transfer function in a different sample time than where it comes from. You can run step(x,0:0.01:10) or step(x,0:0.001:10) without any problem. You can also run a discrete transfer function at different sample time if that discrete transfer function is defined directly (like a simple z delay). Don't mix the two together.

type cd in Matlab command window, you'll see the discrete transfer function and it has a sample time associated with it. BTW, cd is a Matlab command. You'd better avoid using it as a variable.

sadel 2011-6-3