[Scilab-users] Question syslin

[tim at wescottdesign.com]

On 2014-12-20 12:11, Andreas Ladanyi wrote:
>> On Tue, 2014-12-16 at 11:01 +0100, Andreas Ladanyi wrote:
>>> Hi,
>>> 
>>> i am reading the help of scilab to understand the syslin function. In
>>> the help i can see 3 options for the dom parameter. There is an 
>>> option
>>> for a sampled system 'n'.
>>> 
>>> In this help there is no example for a sampled system. So what is the
>>> difference between a discrete system and a sampled system ? What is 
>>> the
>>> difference between an discrete system d and an sampled system n for 
>>> syslin ?
>>> 
>>> In the most examples i read in the web syslin is called with dom='c'.
>> Hey Andreas:
> Hi Tim,
> 
> thank you for your excellent description. Could this be inserted to
> the online help of the syslin function ?
> 
> cheers,
> 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.
>> 
>>      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.
>> 

I'm not one of the maintainers -- just a happy user for over a decade.  
If someone wants to put the above description into the syslin help, 
they're welcome to it.