Correct way to detect Zero crossing

Hi i am implementing a power-electronics circuit that requires a zero current detector I implemented a crude function that checks if the current (through a current sensor) is more than a small quantity or not but due to this the simulation speed decreased drastically.

function y = ZCD(I)
if(I<1e-8)
    y=1;
else
    y=0;
end

Is there a more elegant way to find zerocrossings?

Answer 1

[Jim Riggs]

A thought you might want to consider:

if you multiply the current value, y(i) by the previous value y(i-1), this product is only negative when y(i) has crossed zero.

If y(i) and y(i-1) are both positive, the product is positive, likewise, if they are both negative, the product is positive.

zerocross =   y(i)*y(i-1) < 0 ;

Now, if you care whether it's rising or falling, you need to test the sign on y(i)

if zerocross
    rising  = y(i) > 0 ;
    falling = y(i) < 0 ;  %(or falling = ~rising)
end

You can make this a function :

function zerocross = detectzerocross(x)
persistent last

if isempty(last)   % initialize persistent variable
    last=0;
end

zerocross = x*last>0;  %  = TRUE when x crosses zero
last = x;
end

This function "remembers" the last value of x each time you call it.

Answer 2

[Adam Danz]

The entire function can be replaced with

y = I<1e-8;

which will return a logical (true | false). If you want a double (1 | 0),

y = double(I<1e-8);

Note that this doesn't necessarily identify zero crossings. It merely identifies values less than 1e-8.