Re: Slow varying inputs

On Wed, 2005-03-02 at 22:25 -0500, Daniel Nilsson wrote:
> Hi All,
> 
> I'm working on a project where I'm trying to sample more really slow
> varying inputs, I'm only interested in getting a new value every 500ms
> or so and the data will not vary much in this time period.
> 
> From looking at the Comedi API I find function designed to help out
> sampling such data, though they all have the status deprecated. Why
> are the functions deprecated ? Is there something else that provides
> the same functionality ?
> 
> If not, does anyone have any filter code that you can share the is
> designed to filter out any noise that might be present on the inputs ?
> The issues that I'm having right now is that the inputs don't vary
> much but I'm getting a measurement values that move back and forth
> around the actual value. I assume some of this is digital noise and
> some is analog noise (60Hz noise from the mains for example). I would
> think someone had already written some code to filter out such noise ?
> 
> Any other suggestions would be welcome. I'm trying to measure the
> voltage and current of a Ni-Cd battery that is being discharged
> through a resistor if anyone was curious...
> 
> Thanks

First a little DSP info: If there's noise on your input you'll need to
sample at twice the maximum frequency of the noise in order to digitally
filter the noise.  Otherwise you'll end up with alias errors in your
recorded input.  If this isn't possible then you need to alter your
setup to reduce stray noise or install a hardware filter before the
input to eliminate the higher frequency noise.  

Once you collect the samples it is easy to digitally filter the signal
by either using a commercial program like matlab, open source tools like
python, of writing a small C program.  If you google around on "digital
filtering" you will find programs.  For instance:

http://ccrma.stanford.edu/software/stk/

Received on 2005-03-03Z05:04:18