# Machine precision in simulink

2 min readSep 9, 2021

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I am currently working on simulink model.

One of the equations is

`"L = qf*(XL0+(XL1*alpha_quadrant))"`

where “alpha_quadrant” is constantly zero. “XL0” and “qf” are both constants. The result of the equation, when graphed, should be constantly -11.08, but as it can be seen in the picture the result keeps fluttering.

# ANSWER

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To determine what is happening, a good approach is to log the key signals involved.

Then use MATLAB to investigate the logged values.

I suspect you’ll find that either alpha_quadrant isn’t always zero or qf is not constant or XL0 is not constant.

The differences in values from what you expect might be very small so you could use some techniques like the following.

`format long g  % will shows higher precision than short, but can still hide some differencesa = 0.05;b = a + eps(a);c = a + 2*eps(a);v = [c,a,b]      % the differences will not be visible in the command window% converting to a string with 17 digits will be enough to see differencesmat2str(v,17)% subtracting values is another way to make the differences visible in the% command windowdv = diff(v)max(dv)min(dv)`

The output of this code is:

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