How do I force a zero value in Simscape language

I have the following equation inside my custom Simscape block.

Mloc == sqrt( (Pratio^(gm1/-A.gam) * (1+(gm1/2)*Minf^2) - 1) * (2/gm1) );

I am encountering an issue where Pratio is near 1.0 and Minf is near zero, however rounding errors are causing the value inside the sqrt to become negative. How do I force the value inside the sqrt to remain non-negative? Here are some example numbers showing this situation.

sqrt(((14.715780533017/14.715780064774)^(0.4/-1.4) * (1+(0.4/2)*0.0001079^2) - 1) * (2/0.4))

Answer 1

[Sebastian Castro]

I would start off by changing the equations to something like

let
  X = (Pratio^(gm1/-A.gam) * (1+(gm1/2)*Minf^2) - 1) * (2/gm1);
in
  if X >= 0
    Mloc == sqrt(X);
  else
    Mloc == {0,'appropriate_unit'};
  end
end

In reality, most Simscape language blocks will have some sort of "smoothing" to prevent the sharp discontinuity -- usually a linear ramp down to zero over some threshold range (see, e.g., the Translational/Rotational friction blocks). You can try that further step if you run into problems, though.

- Sebastian

[Ries Smith]

Thank you. That did the trick.