How to implement nested if statements in Modelica?

Question

I am trying to implement a fault inclusive model of an HVAC system. The fault starts at a user defined time, in this case faultTime = 1000. However, the first part of the if statement is not implemented at all. The following is a snippet of code that is relevant to the question

fcuModel FCU;
Modelica.Blocks.Continuous.LimPID PI(k = 300, Ti = 1, yMax = 1, yMin = 1e-4);
parameter Real faultTime = 1000;
// fault modes: 0-normal, 1-fan failed, 2-valve stuck shut...
parameter Integer faultMode = 1;
equation
  connect(PI.u_m,FCU.Ts_zon); // connects zone temperature to PID measurement
  PI.u_s = 21; // set-point for zone temperature
  if time<faultTime then
    PI.y = FCU.val;
    PI.y = FCU.fs;
  else
    if faultMode == 0 then
      PI.y = FCU.val;
      PI.y = FCU.fs;
    elseif faultMode == 1 then
      PI.y = FCU.val;
      FCU.fs = 1e-4;
    end if;
  end if;  

When I simulate, it runs without error but the it directly goes to the equations under faultMode == 1, without simulating the fault-free state for the first 1000 seconds.

Answer 1

I modified your model to make it work directly by introducing some variables and changing some parameters. Which resulted to be:

model FCU

  Modelica.Blocks.Continuous.LimPID PI(k = 0.1, Ti = 1, yMax = 1, yMin = 1e-4);

  parameter Real faultTime = 1000;
  parameter Integer faultMode = 1;

  Real val;
  Real fs;

equation 
  PI.u_s = 21; // set-point for zone temperature
  PI.u_m = 20.9999; // no feedback as no system available  

  if time<faultTime then
    PI.y = val;
    PI.y = fs;
  else
    if faultMode == 0 then
      PI.y = val;
      PI.y = fs;
    elseif faultMode == 1 then
      PI.y = val;
      fs = 1e-4;
    else
     assert(false,"Unknown faultMode");
    end if;
  end if;
  annotation (experiment(StopTime=2000), uses(Modelica(version="3.2.2")));
end FCU;

The result below (simulated in Dymola) seem to be what I would expect.

Hope this helps...