Hi John,
How did you get the yellow colors on the rate transition blocks on the signals coming out of those? If you turned on Sample Time Colors, I wonder why the output of the Discrete Derivative block isn't red.
Anyway, I'm pretty sure what's happening is that the solver is running a at higher rate than 1e-5. The discrete derivative block is updating at a sample time of 1e-5 secs, and at each sample time computes
y_discderiv[n] = (y@rate[n] - y@rate[n-1])/1e-5, where the samples of y@rate are taken at 1e-5.
But the continuous signal y is updated faster (is my assertion), and the output of the Derivative block is essentially
dy/dt = (y(t) - y(tprev))/(t - tprev)
where t is current sim time and tprev is the time of the last major step. If t - tprev < 1e-5 (as I expect), then dy/dt won't match y_discderiv (at the 1e-5 sample points) because they are computed based on different quantities.
If you want them to match, you can change your solver settings to fixed-step with a step size of 1e-5. But there is no guarantee that a step size of 1e-5 is small enough to accurately simulate the continuous plant dynamics (you'd have to check that), keeping in mind that whatever solver settings you have are already using a smaller time step.
