Simscape/Simulink persistent variable in algebraic loop

I have a simple Simscape electrical model containg a circuit that trips if a current is exceeded. I want the trip to happen a certain time after the current excess, so I use Matlab function block and the Simulink Clock Block to track time.

The red highlighted portion is flagged as modifying a persistent variable in an algebraic loop.

This page says to avoid such loops:

https://www.mathworks.com/help/physmod/simscape/ug/limitations.html

But I am not sure how to acheive my goal without the MATLAB function block and the Simulink Clock.

The code in my function block is here:

% Monitors current.  If over current, trip with a timed delay.

function switch_control = current_monitor(current, clockin)
persistent starttime;
persistent tripped;

if isempty(starttime)
    starttime = 0;
end

if isempty(tripped)
    tripped = false;
end


if (current > 10)
    starttime = clockin;
    
    if ((clockin - starttime) > 0.2) % fixed delay of 200ms
        tripped = true;
    end
end

if(tripped)
    switch_control = 0;
else
    switch_control = 1;
end

end

Any insights on what I am doing wrong?

Answer 1

[Matthew Mishrikey]

I think I managed to figure out a solution. As suggested elsewhere a delay block can "break" the algebraic loop.

I needed three persistent variables to make it work, not sure if there is a simpler way.

The timing was off so I changed the solver and also changed it to fixed step with time step siginificantly smaller than the delay I was looking for. Then it worked very well.

Answer 2

[Alan Lian]

I meet the same problem today, and I try a new way different from you. Put a switch after switch control, and use a step to turn on this signal channel in 1e-5 second. This solution works well in my condiction.