Analog Input Fluctuations

[Peter Nachtwey]

Question: Does anyone have a formula or rule-of-thumb for sizing filters for this type of application. I have never had to do this, so I really don't have a clue what to get.
Steve

Yes, 1/(2*PI*R*C) = Cutoff frequency. Where R*C is the time constant. The cutoff frequency should be 5 to 10 times higher than the highest frequency you want to measure. You may reduce the cutoff frequency to remove even more noise but beyond this point it is better to fix the noise problem. The equation allows you to choose R * C. Since the resistor is in series with what ever the input resisteance is, a resistor that is too big will noticeably attenuate the signal. To compensate one usually chooses a big capacitor.

A =-20 log | 1 + jwRC |

 

[danw]

Mr. Jenkins beat me to it. I'd try to isolate the 0-10V signal by isoalting it through a signal conditioner, and converting it to 4-20mA.

But before I'd do that, I'd replace the 10Vdc power supply on principle. Power supplies are problematic, they're the source of many failures and the ability to swap one out in a hurry with another standard power supply is SOP in our plant. Unless there is some control reason that you're using the 10V from the drive, why not use a real power supply?

It wouldn't surprise me if replacing the supply eliminated the 'noise' factor, since it's evident that you have a grounding or more likely, common mode problem between the drive and the PLC input.

Dan

 

[Steve Bailey]

Mr. Jenkins beat me to it.

By a couple of years!

 

[Steve Etter]

Since we are back to this......

I did indeed finally find the source of this problem but neglected to post it....so here is what I found.

The problem turned out to be something within the analog DC drive itself. The manufacturer, who is well aware of this issue, offers and sells an isolation board that solved the problem. Once I installed one, POOF!, no more problem.

As luck would have it, my company had previously been using one of these isolation boards on each and every application of the DC drive for years, but nobody could remember why. During one troubleshooting exercise, I decided that the isolation board was causing more trouble than it was worth, I stopped using them.

Now I know better. I only wish the guys before me had kept some records....

Steve