adccommunitymod (AutomationDirect) asked a question.
Created Date: November 13,2014
Created By: MicroF
**** This post has been imported from our legacy forum. Information in this post may be outdated and links contained in the post may no longer work.****
So I am using three high speed input modules to read 6 frequency signals. I use an averaging function to filter out any noise in the signals. However channels 1 and 2 on each of the cards read differently. Channel one reads the correct frequency when testing with a function generator while channel two reads a larger value. I found that channel two is seeing jumps to around double the actual frequency and can not figure out where this is coming from. The frequency i am reading is between 0 and 5000 Hz, i am testing with a sine wave. when testing with a square wave there are no issues. If anyone can offer some insight as to while this might be occurring that would be greatly appreciated.
Created Date: November 14,2014
Created by: OpZed
I think these are your two clues, "double the actual frequency ", "when testing with a square wave there are no issues "
With the sine wave the signal is changing slowly, so it spends more time near the switching turn on-off voltage. So twice per cycle you could pickup noise. With the square wave the signal is good because the inputs turn on-off quickly and don't bounce at the transitions.
Created Date: November 14,2014
Created by: MicroF
That makes sense, but the issue is never seen on channel one is why this is odd. For testing i send the same signal to both channel one and two of the card, channel one sees the correct frequency while channel two is off because of the doubling.
Created Date: November 15,2014
Created by: MicroF
To anyone that reads this post. I have found out a few things.
First i am aware the cards are not meant to read sine waves. I am only doing this to prepare for the modified sine waves the sensors will output, in case they are closer to a true sine then a square.
The cards appear to work fine for channel one if you filter out values above around 100000 hertz, this is just due to the card not being designed for this. Channel two on the cards have an interesting behavior. the signal is duplicated at different non integer multiples of the frequency. the multiple of the frequency that you will see is dependent on input voltage and not frequency. as the voltage approaches 1 volt the multiples approach the true frequency until the voltage is too low to read the signal.
This behavior would not be very interesting if it was the same on channel one, but is not. We have tried this with 6 cards and get the same behavior every time.
Created Date: November 17,2014
Created by: MicroF
Update. Found the source of the "noise ". The Cmore touch screen display we are using is interfering with the HSI cards or the timing of the unit. I noticed that all my signals were going to 0 at a periodic interval so I varied the voltage and frequency of my signals. This had no change on the period of the 0 input. Then I thought that maybe the serial cable was interfering so i unplugged it. the moment the cable was unplugged all the signals became clean.
Whether it was some form of interference from the cable or a disruption in the CPU timing I do not know. I am always open to insight from anyone else.
Created Date: November 18,2014
Created by: kewakl
does your cmore have ethernet?
If so, try using it. See if the noise is the same, reduced or eliminated.
Created Date: November 18,2014
Created by: MicroF
It does not. So I am forced to use the serial cable.
Created Date: November 18,2014
Created by: kewakl
Is the serial cable from AD or is it fabricated?
Shielded? Grounded?
Created Date: November 18,2014
Created by: Do-more PE
Is the C-more itself grounded?
Created Date: November 18,2014
Created by: MicroF
Its a shielded cable from AD and I have grounded the C-More to the cabinet(which is connected to the AC ground)
Created Date: November 13,2014
Created by: MicroF
So I am using three high speed input modules to read 6 frequency signals. I use an averaging function to filter out any noise in the signals. However channels 1 and 2 on each of the cards read differently. Channel one reads the correct frequency when testing with a function generator while channel two reads a larger value.
I found that channel two is seeing jumps to around double the actual frequency and can not figure out where this is coming from.
The frequency i am reading is between 0 and 5000 Hz, i am testing with a sine wave. when testing with a square wave there are no issues.
If anyone can offer some insight as to while this might be occurring that would be greatly appreciated.