Hi all!
Maybe someone here might help me with a DSP problem that should be
trivial, but to me, as a newbie to DSP, is not.
I'm developing a microcontroller-based speed controller for my small
hydroelectric plant. I'm using a PIC 16F628 for this. I'm detecting the
zero crossings of the 50Hz signal coming from the generator, and measuring
the duration of each half cycle, obtaining a signal at a rate of 100
samples per second. The signal is in 16 bit format, and nominally 10000
counts tall. Typical variations are a few tens of counts, and occasionally
as much as 1000 counts. I'm subtracting the 10000 reference, and using the
resulting error signal as input to a pro****tional-integral control
function, whose output drives dump loads that burn off the excess output
from the generator.
The little problem I have is that a small 420Hz signal on the power line,
apparently coming from the generator's internal voltage regulator, is
causing jitter in the zero crossings, which is in turn causing the
pro****tional function of my controller to imprint an unwanted modulation
on
its output. So I would like to apply a low pass filter function to my
signal, with a cutoff frequency of roughly 20Hz, before the P-I function.
The question is: How can I implement this DSP low pass filter? It has to
be in some simple way, because neither the PIC nor I can handle overly
complex math!
The only thing I can come up with is averaging the last several samples,
perhaps with some weighing. But there must be some better method!
It is desirable that the time delay in the filter be as short as
possible.
I would be most grateful for any help in this, be it with an explanation
of how to implement the filter, or a hint as to where I can find this.
Manfred Mornhinweg
http://ludens.cl
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