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Incorporating different sampling rates.....

Started by MSCS039 November 20, 2003
Hello,

I have a couple of questions regarding control system simulation:

-- How could I incorporate different sampling rates in a single feedback
system using Simulink ? Can simply having different 'sample time' values
(relatively prime) in different blocks solves the problem ?

-- Actually I want to simulate a feedback system (multirate) where outer
loop time period is an integer multiple of inner loop period. On which rate
to discretize the plant (using 'c2d')?

-- Has anybody worked on 'lifting a discrete/continuous time signal/system'
using Matlab/Simulink ? If yes, then please tell me how ..... (any example).

Thanks to all respondents in advance

Regards,

Sarmad



Message
See comments interspersed with your questions.
-----Original Message-----
From: MSCS039 [mailto:m...@nu.edu.pk]
Sent: Thursday, November 20, 2003 10:32 AM
To: 'm...@yahoogroups.com'
Subject: [matlab] Incorporating different sampling rates.....

Hello,

I have a couple of questions regarding control system simulation:

--  How could I incorporate different sampling rates in a single feedback
system using Simulink ?  Can simply having different 'sample time' values
(relatively prime) in different blocks solves the problem ?
 
Yes, you can mix blocks with different sample rates, even if they aren't harmonically related.  However, as part of your system design you must decide how you are actually going to merge these disparate input rates.  Are you just using a sample-and-hold?  If so, Simulink will do fine with just different sample rates in the different blocks.  If you need better quality, you'll have to change sample rates using upsampling/downsampling (a.k.a. resampling).

--  Actually I want to simulate a feedback system (multirate) where outer
loop time period is an integer multiple of inner loop period. On which rate
to discretize the plant (using 'c2d')?
 
It depends.  If the plant control signal(s) is(are) at the inner rate only, use the inner rate.  If the plant control signal(s) is(are) at the outer rate only, use the outer rate.  If the control signal(s) can change at either rate, you must either use the highest common multiple frequency, if it exists; or if the highest common multiple frequency doesn't exist, you'll have to leave the system as continuous-time.  The highest common multiple frequency of 1Hz and 5.5Hz, for example, is 11Hz.  The highest common multiple frequency of 1Hz and sqrt(2)Hz doesn't exist.  If the highest common multiple frequency is extremely high compared to the highest-frequency pole/zero in the plant, you may save execution time going with the continuous-time system, but in most circumstances the discrete-time system will simulate much more quickly.
 
--  Has anybody worked on 'lifting a discrete/continuous time signal/system'
using Matlab/Simulink ? If yes, then please tell me how ..... (any example).
 
Sorry, I'm not sure what you mean here, so I'm pretty sure I haven't done it and cannot answer this question off the top of my head.  Anybody else?
 
Sincerely,
Glen Ragan
 
Thanks to all respondents in advance

Regards,

Sarmad

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