# Re: Sampling time

**Ali Khalaf** (*hkila@batelco.com.bh*)

*Sat, 22 Feb 1997 14:02:18 +0100*

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.*

*> *

*> Regards, Jan Jantzen*

*> *

*> Technical University of Denmark | Tel +45 4525 3561 (direct)*

*> Att: Jan Jantzen | Fax +45 4588 1295*

*> Dept. of Automation, Bldg. 326 | Jantzen@iau.dtu.dk*

*> DK-2800 Lyngby, DENMARK | http://www.iau.dtu.dk/~jj*

*> *

*> 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 ?