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simulink, switch problem

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aft
aft 2011-7-30
回答： Josco Kester 2019-1-10
Hi, a little problem i m facing while using simulink. my time step is of 0.025s and i have a switch which says enter into my controller when t > 0.075, meaning that at t=0.1s onwards. Simulink does the simulations at the time values 0, 0.025, 0.05, but then 0.07500000000001 and enters into my controller. WHY ??? I am having an algebraic loop in my system, does it have anything to do with that ? but if i change my switch condition to 0.1 instead of 0.075, no problem.
thanx in advance,
khawaja

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回答（3 个）

Fangjun Jiang
Fangjun Jiang 2011-7-30
I am not sure. Maybe you don't need to care about it. It might be floating point number precision thing, such as:
>> (0.025*3)==0.075
ans =
0
>> (0.025*4)==0.1
ans =
1
Prashant Srivastava
For algebraic loops error, you can connect a unit delay block between your input and the output(that you're reusing).
Josco Kester
Josco Kester 2019-1-10
@aft, your problem exists because the duration of the time steps in Simulink unfortunately is NOT exact. This is caused by floating point round-off (see https://blogs.mathworks.com/simulink/2012/02/09/using-discrete-data-as-an-input-to-your-simulink-model/ ).
A work around in your case could be to use t > 0.08 as switch criterium. In any case avoid any exact criterium such as t > n*0.025. How that criterium behaves is NOT determined, because of the floating point round-off error. The criterium t > 0.1 could also give a problem next time...

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