All branches in if equation with non-parameter tests must have the same number of equations - Modelica

Question

I get an error on Modelica saying:

All branches in if equation with non-parameter tests must have the same number of equations

The source of the error is following section of the code:

equation
  if der(Posit2.s)<=0 then
    pressure=4e5+((500e5-4e5)/0.0111)*(0.0111-Posit2.s);
  end if;

Do you know how to deal with this error?

Answer 1

You need an else, so the obvious idea is to say that pressure doesn't change:

 equation
    if der(Posit2.s)<=0 then
           pressure=4e5+((500e5-4e5)/0.0111)*(0.0111-Posit2.s);
    else
           der(pressure)=0;
    end if;

However, this will likely not compile due to the index-problem. One possibility is to do manual index reduction and write something like:

 initial equation
    if der(Posit2.s)<=0 then
           pressure=4e5+((500e5-4e5)/0.0111)*(0.0111-Posit2.s);
    else
           pressure=4e5;
    end if;
 equation
    if der(Posit2.s)<=0 then
           der(pressure)=((500e5-4e5)/0.0111)*(-der(Posit2.s));
    else
           der(pressure)=0;
    end if;

Note that this equation has der(pressure)=...*der(Posit2.s); - due to the manual index reduction.