Hello,

In developing a piece of software which interfaces with FMUs created by OpenModelica, I have come across some literature that stipulates an fmiStatus of fmiDiscard should be returned under circumstances where a result was not convergent.  According to my research, when this happens, I must reduce the step size and try the step again.

Under what circumstances will an FMU created by omc return an fmiDiscard, and what steps can I take to ensure simulation proceeds correctly?

Thanks in advance.

(OpenModelica v1.9.1(r22929))

Hello all,

For the purpose of exploring OpenModelica's code generation for conditional statements, I have created the short model below.  The purpose is to use if-else-end for conditionally evaluated statements in a compiled FMU.

model IfTest

  parameter Real a = 1;

  parameter Real b = 2;

  parameter Boolean useA = true;

  output Real y;

equation

  if useA then

    y = a;

  else

    y = b;

  end if;

end IfTest;

Upon instantiation, I see the if statement has been flattened, resulting in the same path of execution regardless of the value of useA.

class IfTest

  parameter Real a = 1.0;

  parameter Real b = 2.0;

  parameter Boolean useA = true;

  output Real y;

equation

  y = a;

end IfTest;

This isn't ideal, so let's see what happenes if no default value is given for useA.

  parameter Boolean useA;

Instantiated:

class IfTest

  parameter Real a = 1.0;

  parameter Real b = 2.0;

  parameter Boolean useA;

  output Real y;

equation

  if useA then

    y = a;

  else

    y = b;

  end if;

end IfTest;

Great!  The conditional is going to be evaluated differently depending on the value of useA!  Having a quick look at the code of IfTest_06inz.c to confirm:

/*

 equation index: 1

 type: SIMPLE_ASSIGN

 useA = false

 */

void IfTest2_eqFunction_1(DATA *data)

{

  const int equationIndexes[2] = {1,1};

  $PuseA = 0;

}

/*

 equation index: 2

 type: SIMPLE_ASSIGN

 y = if useA then a else b

 */

void IfTest2_eqFunction_2(DATA *data)

{

  const int equationIndexes[2] = {1,2};

  $Py = ($PuseA?$Pa:$Pb);

}

Uhoh... the false branch of equation 2 will always be executed for initialization...  Ok, what if I set useA to use (fixed=false)?

  parameter Boolean useA(fixed = false);

Instantiated:

class IfTest

  parameter Real a = 1.0;

  parameter Real b = 2.0;

  parameter Boolean useA(fixed = false);

  output Real y;

equation

  if useA then

    y = a;

  else

    y = b;

  end if;

end IfTest;

Good, the conditional is still there.  C:

/*

 equation index: 1

 type: SIMPLE_ASSIGN

 useA = $_start(useA)

 */

void IfTest_eqFunction_1(DATA *data)

{

  const int equationIndexes[2] = {1,1};

  $PuseA = $P$ATTRIBUTE$PuseA.start;

}

/*

 equation index: 2

 type: SIMPLE_ASSIGN

 y = if useA then a else b

 */

void IfTest_eqFunction_2(DATA *data)

{

  const int equationIndexes[2] = {1,2};

  $Py = ($PuseA?$Pa:$Pb);

}

Great, now I can use the conditional by setting the start value before initialization (fmiSetBoolean() would do the trick I believe).

This leads to a three-part question:
  1. Why is the conditional flattened when a default parameter is provided, as in case 1 above?
  2. Why are parameters zero-initialized (overwriting any fmiSetxxx) in 06inz.c if no default values are provided?
  3. Is the use of (fixed=false) here the right solution for the desired behaviour?

Note that I could have used the equation

y = if useA then a else b;

for this single-line assignment, but want to later extend the concept to more complex conditional blocks in the FMU.

Thank you in advance!
Mike