Re: Sampling time

Ali Khalaf (hkila@batelco.com.bh)
Sat, 22 Feb 1997 15:03:04 +0100


Jan Jantzen wrote:
>
> On Thu, 20 Feb 1997, Ali Khalaf wrote:
>
> > Jan Jantzen wrote:
> > >
> > > It should be chosen somewhat smaller than the dominating time constant in
> > > the process, for instance between 1/10 and 1/5 of the time constant.
> > > If the sample period is too short, the computation of CE will become
> > > too sensitive to noise. This normally shows up as a restless control
> > > signal. If CE happens to be implemented as a difference, several
> > > gains depend on the sample period; a change in sample period must then be
> > > followed by a compensation in a gain factor to keep the proportional,
> > > integral and derivative gains intact.
> > >
> > > The sample period can also be related to the derivative gain Td. Astrom
> > > and Wittenmark (1984) suggest that the sample period should be between
> > > 1/10 and 1/2 of Td. In connection with the Ziegler-Nichols rules,
> > > this implies that Ts should approximately equal 1-5 percent of the
> > > ultimate period Tu.
> > >
> > > On Thu, 20 Feb 1997, Ali Khalaf wrote:
> > >
> > > > What is the required sampling rate required for a fuzzy logic
> > > > controller. I am mainly concerned with the application of fuzzy
> > > > controller in power system stabilizers. Also what effect does varying
> > > > sampling rate have ?
>
> > But if we take the sampling time as 1/10 to 1/5 of the dominant time
> > constant ,wouldn't that lead to ignoring the transients ?
>
> I suppose that could happen. Another rule of thumb is to
> have 2-4 samples during the rise time (Astrom & Wittenmark,
> 1984, p 61).

In some of the literature , the fuzzy logic controller uses the error
e(k) and the CE=e(k)-e(k-1) . In others fuzzy logic controller uses the
error e(k) and CE=(e(k)-e(k-1))/Ts , where Ts is the sampling time .
What are advantages and disadvatges of both approaches .