Simulink, ODE15S and stiff-system solutions getting unnecessarily stuck.

[Kelvin Hales]

I find when solving my present stiff Simulink 5.1 (R13SP1) system with
ODE15s, that the solution tends to grind to a halt when approaching a
steady-state, i.e. most annoyingly at the very point where it should
start to take bigger steps and zoom along. If, however, I stop the
simulation and re-start it by initialising the states with the end
state of the previous run, then the solution immediately picks-up and
shoots along.

Any ideas anyone?
Its as though the solver is getting bogged down in the detail because
it cannot forget the previous small time-steaps.
A problem with a zero-crossing detection, maybe?
Is there something in the Simulink debugger/profiler that would reveal
what's happening?

Kelvin B. Hales
Kelvin Hales Associates Limited
Consulting Control Engineers
Web: www.khace.com

[Mike Tocci]

Hi Kelvin,

I'm not 100% sure, but it could be zero-crossings that are causing
this. Whenever you stop a simulation and use the final state as an
initial state, there is some information not stored in the state
vector that is lost. This could be affecting the zc detection in
some blocks.

I do recommend using the Simulink debugger to try to find the problem.
If you use the command-line debugger (sldebug), there are commands to
print out solver information. I'd try setting a time break point
(tbreak) so you get to the point where the model nears steady state,
and then use the command "strace 4" to print out all the information
in the solver. If there's any information in the output that's not
clear, let me know and I'll see if I can help. There can be alot of
information printed to the screen, so I recommend taking a few time
steps using "step top" to see if you can tell why the time steps are
so small.

Hope that helps,

Mike

[J]

Mike Tocci <mto...@mathworks.com> wrote in message <clu67s$53d$1...@fred.mathworks.com>...

I too have the same issue. I'm simulating a highly dynamic system with lots of power setpoint jumps with ODE15s. The model matches all the extreme dynamic changes fine until a large drop in power after being held at a steady state value. At this large drop in power, the model starts to progress at a snail pace. The step size it progresses at is at about 8e-8 seconds.

I am playing around with the minimum step size, maximum step size, relative error tolerance, and absolute error tolerance, but nothing is getting me past this point.

I am attempting to use a fixed step solver, but am not partial to this because it is so slow.

I will look into the suggestion posted by Mike. Any other suggestions are appreciated.

Thanks,

J