scicos newbie question: when i loop back simulation fails with message "algebraic loop"

[schuffe]

Hi,

any advice for my newbie question would be appreciated. here is what i
do,
sine wave generator ==> summation (+) ==> gain ==> POINT A
from Point A i connect back to summation and to a plot block.
Simulation
comes up with error "algebraic loop". what is wrong?

Thanks,
Schuffe

[Serge Steer]

schuffe a �crit :

to understand the problem suppose that the output of summation (+) is
the variable y at point A the signal value is gain*y so the second input
of summation is gain*y and the outut of summation is sin(t)+gain*y and
it should be equal to y. It is the reason why you got an "algebraic
loop" error. In this simple case it should be solved by the simulator
(or manually with y=sin(t)/(1-gain)) but in many cases such algebraic
loops may be costly or impossible to solve.

[RRogers]

Try inserting some delay in the closed loop you have; say in the gain
block or possibly in the "connect back". You are asking a dynamic
simulator to solve a DC recursion.
You should be able to make it small enough not to matter. In fact
there should be some in the gain anyway in a dynamic system.
You might also write out the algebra and solve the recursion (i.e. y=f
(input,y)-> y=g(input)) by hand and then use that as the output block.