Analog signal filtering

Plc_User

Member
Join Date
Dec 2005
Location
Belgium
Posts
317
Hey,

I've read some threats here about analog signal filtering. Most propose a first order filter. I've done some experiments with first order filters and experienced best results with cascading first order filters. Especially towards delay time of the filterd signal. It was better to cascade f.i. three filters with a moderate filter constant then to use a high filter constant on one filter.
I am not experienced in the theoretical principles of filters, I wondered if some people used more sophisticated filters (like Butterworth or so) and what their experiences were ?
Thanks.
 
Plc_User said:
I wondered if some people used more sophisticated filters (like Butterworth or so) and what their experiences were ?
Thanks.

We use 4th order Butterworth filters in our motion controllers. Implementing a second order Butterworth filter is just a little harder than the cascaded single pole filters, but the Butterworth filter has a higher attenuation per pole.

ftp://ftp.deltacompsys.com/public/PDF/Mathcad - Butterworth.pdf

You must be careful about how the phase shift will affect your data at all frequencies of interest. As you have noticed, you may be better off using a 4 pole filter with a higher corner frequency than a single low pass filter, but each stage still add phase lag. From the graphs you can see the phase shift starts to become noticeable at about 1/10th of the corner freqency.
 
The reason that first order filters get recommended so much is because they are relatively easy to implement and tune. Most of these filter recomendations are for smoothing out a data display on an HMI or something similar. Not too demanding stuff. This doesn't mean they are necessarily the best choise. As Peter points out there are other filter designs that are more selective that are also relatively easy to implement.


Here are a couple of websites I found when looking for filter information. They may be of interest:

http://www.maxim-ic.com/appnotes.cfm/appnote_number/733
http://www.freqdev.com/guide/analog.html
http://www.evaluationengineering.com/archive/articles/0599filt.htm

Keith
 
Something to think about

That freqdev.com got me thinking.
If the ladder filter samples every 10 milliseconds (100Hz), what prevents aliasing? A 200 Hz signal would suffer from aliasing UNLESS the analog input cards have a filter to reduce the frequency response down to about 50Hz. Now the problem is that the analog input card can not respond to quickly changing inouts.

Are analog cards configurable? Can the anti aliasing filters be programmed? If they don't have anti aliasing filters then oops.

I don't deal with analog input on the PLCs, just on the motion controllers. So inquiring minds want to know.
 
The AB 1756-IF8 has two filter fields. The first is a module-wide filter that allows the user to select a -3dB filter frequency from a list of 6 frequencies (10, 50, 60, 100, 250, 1000). They don't get into filter implementation so I'm not sure what this really is.

In addition to this you get a first order low-pass filter for each channel with an independently adjustable time constant. The low end of this time constant is limited by the configurable channel sample rate.

I'm not sure about other manufacturers. I don't get around much anymore.

I would hope that the module-wide filter would be an anti-aliasing filter but there is no guarantee of that at this point. I have used stand-alone hardware anti-aliasing filters in the past. I those cases the inputs didn't contain any anti-aliasing filters and that was causing me grief.

Keith
 

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