Am 16.12.2014 um 18:54 schrieb Tim Wescott:
Hey Andreas:
Domain = 'c' is for a continuous-time system, where the system is
defined as
dx/dt = A * x + B * u,
y = C * x + D * u
This is a normal continuous-time linear state-space system description.
Domain = 'd' is for a discrete-time system where the sampling interval
is left undefined for whatever reason (in my case, it's usually because
I'm being lazy, but sometimes it's because the sampling interval isn't
constant, or because there's no meaningful "sampling interval" in the
problem).
It defines the sampled-time system:
x{k} = A * x{k-1} + B * u{k}
y{k} = C * x{k-1} + D * u{k}
(Note the mixed time indexes on the input and state variables on the
right-hand side of these equations. You'll sometimes see this expressed
differently, so if you're trying to implement something from an article
or book, pay attention!)
Domain = n is for a discrete-time system where the sampling interval is
defined. It works exactly like domain = 'd', except that things that
depend on the real-world frequency, like Bode plots, will come out
right.
At this point i am a little bit confused. How does syslin works when the
sampling interval is not defined (dom=d) ? And why the bode plots come
out wrong ? What syslin is doing wrong if dom=d ?
When I am doing control system design this is usually the form that I
use, because by the time I'm down to this level of detail I've usually
established the sampling rate, and I'm working at tuning the system to,
or verifying it against, some real-world criteria that must be expressed
in the frequency domain.
cheers,
Andy
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