id	summary	reporter	owner	description	type	status	priority	milestone	component	version	resolution	keywords	cc
1110	DAE equation generated by OMC 1.4.5 seems to be wrong	Schubert TUD	Schubert TUD	"Hello everybody,

here a problem I also reported in the OpenModelicaInterest mailing list.

When trying to simulate the following model, a different set of equations (neglecting v) is generated as C-Code.

model FlatTest
  Real pb(start = 12000000);
  Real Qb;
  Real pa(start = 12000000);
  Real Qa;
  parameter Real s = 1;
  input Real v;
  input Real inFlow;
  
equation
  der(pb) = (Qb - v) / s;
  der(pa) = (Qa - v) / s;
  pa = pb;
  inFlow + Qa + Qb = 0;
  
end FlatTest;

Looking at the C-code generated by the OMC given below, one can reconstruct that the following (incorrect) system of linear equations has been implemented by the OMC:

der(pa) -s/s^2*Qa = 0;		<- v is missing !
-der(pb) + der(pa) = 0;
-s/s^2*Qb + der(pb) = 0;	<- v is missing !
Qb + Qa = -inFlow;

Strangely, when you turn the ""parameter Real s=1"" into a ""constant Real s =1"" everything is fine. The system of equations then reads:

der(pa) - Qa = -v;
-der(pb) + der(pa) = 0;
-Qb + der(pb) = -v;
Qb + Qa = -inFlow;

Kind regards,

Christian Schubert


******************************
* C-Code: (FlatTest.cpp)     *
******************************
int functionODE()
{
  state mem_state;
  mem_state = get_memory_state();
  declare_matrix(A34,4,4);
  declare_vector(b34,4);
  set_matrix_elt(A34,0, 3, 4, ((-$s) / ($s * $s)));
  set_matrix_elt(A34,0, 2, 4, 1.0);
  set_matrix_elt(A34,1, 1, 4, -1.0);
  set_matrix_elt(A34,1, 2, 4, 1.0);
  set_matrix_elt(A34,2, 0, 4, ((-$s) / ($s * $s)));
  set_matrix_elt(A34,2, 1, 4, 1.0);
  set_matrix_elt(A34,3, 0, 4, 1.0);
  set_matrix_elt(A34,3, 3, 4, 1.0);
  set_vector_elt(b34,0, 0.0);
  set_vector_elt(b34,1, 0.0);
  set_vector_elt(b34,2, 0.0);
  set_vector_elt(b34,3, (-$inFlow));
  solve_linear_equation_system(A34,b34,4,34);
  $Qa = get_vector_elt(b34,3);
  $der$lPpa$rP = get_vector_elt(b34,2);
  $DER$pb = get_vector_elt(b34,1);
  $Qb = get_vector_elt(b34,0);
  restore_memory_state(mem_state);
  return 0;
}"	defect	closed	high		Simulation Code Generation	1.4.5	fixed		Schubert TUD