MATLAB Central Newsreader - variable time step in simulink

variable time step in simulink

Omni Domni — Sun, 13 Apr 2008 16:51:02 -0400

Hello,

I have a simple subsystem that adds two numbers, A and B.

While A and B change in a continuous way, I need the
output (that is A+B) to be updated in a discrete rate,
that varies as the simulation progresses.

If I wanted a constant rate (say 5secs) using a 'sum'
block with 'sample time = 5' would be enough. Is there any
way to change the sample time dynamically?

Re: variable time step in simulink

Phil Goddard — Mon, 14 Apr 2008 03:23:02 -0400

Can you explain how the model determines when the next
sample rate will be?
Is it based on a random number? On the comparison of two
(or more) signals? Something else?
Knowing this should help in giving you some ideas on how to
proceed.

Phil.

Re: variable time step in simulink

Omni Domni — Tue, 15 Apr 2008 20:12:02 -0400

"Phil Goddard" <philgoddardNOSPAM@telus.net> wrote in
message <ftuimm$1bo$1@fred.mathworks.com>...
>
> Can you explain how the model determines when the next
> sample rate will be?
> Is it based on a random number? On the comparison of two
> (or more) signals? Something else?
> Knowing this should help in giving you some ideas on how
to
> proceed.
>
> Phil.

Thanks for answering Phil.
To be more specific, my problem has to do with a control
system. When a change is detected in the controlled
variable the control system responds to it after an
initial time delay say 10 seconds.
If the controlled variable hasn't been restored to the
desired value after the first action, the following
control system responces should be in time intervals of,
say 2 seconds. The rest of the system runs in continuous
time.
My first solution was definitely not good, so I'm
searching for a better way to implement such a kind of
systems in simulink. I hope I haven't missed any reset-
enabled triggered time-counting blocks.

Re: variable time step in simulink

Phil Goddard — Tue, 15 Apr 2008 23:50:19 -0400

Your problem doesn't sound too difficult, although giving
you a specific answer without seeing the exact model is
difficult.

You're going to have to use some combination of Triggered
and Enabled subsystems from the "Ports & Subsystems"
library, possibly in conjunction with one of the
counter/timers within the "Sources" library.
Some of the flow control blocks like "If" and "If Action"
subsystems may also come into play.

It sounds as if you may have already done that, although in
a manner that you consider messy.
The next step may be an S-function (although I don't think
you're going to need to do this).

Phil.