OpenModelica complains about a negative value which can't be negative - modelica

Question

Following this question I have modified the energy based controller which I have described here to avoid negative values inside the sqrt:
model Model
//constants
parameter Real m = 1;
parameter Real k = 2;
parameter Real Fmax = 3;
parameter Real x0 = 1;
parameter Real x1 = 2;
parameter Real t1 = 5;
parameter Real v0 = -2;
//variables
Real x, v, a, xy, F, vm, K;
initial equation
x = x0;
v = v0;
equation
v = der(x);
a = der(v);
m * a + k * x = F;
algorithm
if time < t1 then
xy := x0;
else
xy := x1;
end if;
K := Fmax * abs(xy - x) + k * (xy^2 - x^2) / 2;
if abs(xy - x) < 1e-6 then
F := k * x;
else
if K > 0 then
vm := sign(xy - x) * sqrt(2 * K / m);
F := Fmax * sign(vm - v);
else
F := Fmax * sign(x - xy);
end if;
end if;
annotation(
experiment(StartTime = 0, StopTime = 20, Tolerance = 1e-06, Interval = 0.001),
__OpenModelica_simulationFlags(lv = "LOG_STATS", outputFormat = "mat", s = "euler"));
end Model;
However, it keeps giving me the error:
The following assertion has been violated at time 7.170000
Model error: Argument of sqrt(K / m) was -1.77973e-005 should be >= 0
Integrator attempt to handle a problem with a called assert.
The following assertion has been violated at time 7.169500
Model error: Argument of sqrt(K / m) was -6.5459e-006 should be >= 0
model terminate | Simulation terminated by an assert at the time: 7.1695
STATISTICS 
Simulation process failed. Exited with code -1.
I would appreciate if you could help me know what is the problem and how I can solve it.

Answer 1

The code you created does event localization to find out when the condition in the if-statements becomes true and/or false. During this search it is possible that the expression in the square-root becomes negative although you 'avoided' it with the if-statement.
Try reading this and to apply the solution presented there. Spoiler: It basically comes down to adding a noEvent() statement for you Boolean condition...