Ok I understand why the program with cpt is looping.

But why the omc compiler is accepting the first program? Should I report this bug?

Hello,

I don't understand why it is possible to call pre() during integration time. With omc 1.9.1+dev (r18345) I can simulate equation

  Real x (start=0);

  x = pre(x) + 1;

where I observe that the growth of x depends on the numberOfIntervals parameter, which seems odd to me, I would expect the compiler to reject this program.

My other question is on the use of pre() in a discrete context. Does anyone know why equation

  Integer cpt (start=0);

  cpt = if time <= 10 then pre(cpt) + 1 else pre(cpt);

returns

ERROR: Too many event iterations.

?

Thank you,
Antoine

Hello,

I am experimenting with index reduction and conditionals in OM. I understand that during continuous integration, the same if-branch is evaluated and that a new if-branch may only be selected at zero-crossings. However I wonder how an if-branch is selected at inital time. In particular I simulated the following model:

model mode

  Real x(start=0),y(start=3);

equation

  der(x) = y - 3;

  0 = if x < 0 then y else x;

end mode;

Since x = 0 at initial time, the DAE at initial time should be

  der(x) = y - 3;

  0 = x;

and I would expect OM to do index reduction.

However I found that OM is using the if-equation to solve for y:

Variables (2)

========================================

1:  y:VARIABLE(start = 3.0 ) .mode, .Real type: Real 

2:  x:STATE(1)(start = 0.0 ) .mode, .Real type: Real 



Equations (2, 2)

========================================

1/1 (1): der(x) = -3.0 + y

2/2 (1): 0.0 = if x < 0.0 then y else x



Matching

========================================

2 variables and equations

var 1 is solved in eqn 2

var 2 is solved in eqn 1

and I get the following simulation plot:


So how come OM initially selected the first if-branch without checking the conditional?

I use OpenModelica Compiler 1.9.0 beta4+dev (r15987).

Thanks in advance for your insights,
Antoine

How OpenModelica decide to dynamically change of state variables? Does the solver tells him that the system has become singular?

The problem indeed may come from the dynamic state selection because if I run the simulation on the underlying ODE with setIndexReductionMethod("uode") we observe no strange results except that x and y are drifting from the analytical solution (but this is normal since we only use the second order derivative of the constraint x^2 + y^2 = L^2):


I forgot to mention that I use tolerance=1e.-6.

Yes thank you  I've read this paper some times ago, I should read it
again.

The compiler tells me that there are two state sets {x,y} and
{vx,vy}. And from the log of dynamic state selection we can see that
the solver either pick {x,vx} or {y,vy} as state variables. Here's an
excertp:
LOG_DSS           | info    | select new states at time 0.320000|                 | |       | | select vx
LOG_DSS           | info    | select new states at time 0.320000|                 | |       | | select x
LOG_DSS           | info    | select new states at time 0.640000|                 | |       | | select vy
LOG_DSS           | info    | select new states at time 0.640000|                 | |       | | select y
LOG_DSS           | info    | select new states at time 1.280000|                 | |       | | select vx
LOG_DSS           | info    | select new states at time 1.280000|                 | |       | | select x
LOG_DSS           | info    | select new states at time 1.440000|                 | |       | | select vy
LOG_DSS           | info    | select new states at time 1.440000|                 | |       | | select y
LOG_DSS           | info    | select new states at time 2.080000|                 | |       | | select vx
LOG_DSS           | info    | select new states at time 2.080000|                 | |       | | select x

I use version 1.9.0 beta4+dev (r15987).

Hello,

I'm currently experimenting with the standard planar pendulum model in OpenModelica and would like to understand why I get the following strange results.

Here is the model:

model pendulum

  parameter Real g = 9.81;

  parameter Real L = 0.5; // length

  parameter Real m = 1.0; // mass

  Real x(start = L); 

  Real y(start = 0.0);

  Real vx;

  Real vy;

  Real T; // tension

equation

  der(x) = vx;

  der(y) = vy;

  m * der(vx) = - T * x / L;

  m * der(vy) = - T * y / L - m * g;

  x^2 + y^2 = L^2;

end pendulum;

If I ask for simulation with stopTime=50 and numberOfIntervals=500 I get the following expected result:


But if I ask for simulation with stopTime=80 and numberOfIntervals=500 I get this strange result:


By increasing to numberOfIntervals=5000 with stopTime=80 the simulation seems to be again as expected:


Can someone explain to me the meaning of the numberOfIntervals parameter and why I get strange results in the second plot? Note that in both cases, the number of steps taken by the DASSL solver is approximately the same (~10300).

Thank you,
Antoine