LabVIEW
What am I doing wrong with this filter (counter/frequency issue), probably another newb question.
Hello Ralph,
I'm not sure about mounting your sensor to your rig, but I can provide a couple ideas about the filtering. Depending on the type of noise, the digital filters on the PCI-6602 could help eliminate the behavior you are seeing. If the noise manifests as a "glitches" or a bouncing signal, you could use another counter with a minimum period to help eliminate the noise. This concept is discussed in greater detail in this KnowledgeBase. I noticed that you are using NI-DAQmx; the practical application of the digital filters on the PCI-6602 in NI-DAQmx is discussed in this KnowledgeBase. A more detailed description of the behavior of these filters is provided in the NI-DAQmx Help (Start>>All Programs>>National Instruments>>NI-DAQ) in the book entitled "Digital Filtering Considerations for TIO-Based Devices".
I also wanted to comment on your original post and explain why you were receiving error -20020. For convenience, I have copied the text of the error code below.
I think you may have misunderstood exactly what the Filter express VI does. The Filter express VI takes a series of values in a waveform and performs filtering on those signals. So, it will look at a waveform made up of X and Y values and apply the defined filter to this waveform. Specifically in your application, the cut-off frequency (fc) is the Upper Cut-Off level that you specified in the Filter express VI; any frequency components of the waveform outside of the range you have defined will be filtered. The fs is the sample rate based on the data that you wire to the Signal input of the Filter express VI. In your VI, this data is coming from the DAQ Assistant. So, fs will be the sample rate of the DAQ Assistant and is associated with the rate at which points are acquired. The sample rate does NOT relate to the bandwidth of the signal, except that the Nyquist theorem tells us that the sample rate must be at least twice the signal bandwidth in order to determine periodicity of the signal. So, in this case, the sample rate of the DAQ Assistant would need to be at least double the high cut-off frequency.
However, you are performing a frequency measurement using a counter. I think this is where the confusion comes in. For the frequency measurement using a counter, the DAQ Assistant returns a decimal value which represents the rate of the pulse train being measured by the counter. It does not return the actual waveform that is being read by the counter. It is this waveform that would be band-pass filtered to eliminate frequency content outside of the filter's bandwidth. Instead of the Filter express VI, I would recommend that you simply discard values that are outside the range you specify. I have modified the code you posted earlier to perform this operation. The image below shows the changes that I made; rather than using the Filter express VI, I simply compare the frequency to the "Low Threshold" and "High Threshold". I use a Case structure to display the value on if it is within the limits. Otherwise, I display a "NaN" value. I have also attached the modified VI to this post.
I'm not sure about mounting your sensor to your rig, but I can provide a couple ideas about the filtering. Depending on the type of noise, the digital filters on the PCI-6602 could help eliminate the behavior you are seeing. If the noise manifests as a "glitches" or a bouncing signal, you could use another counter with a minimum period to help eliminate the noise. This concept is discussed in greater detail in this KnowledgeBase. I noticed that you are using NI-DAQmx; the practical application of the digital filters on the PCI-6602 in NI-DAQmx is discussed in this KnowledgeBase. A more detailed description of the behavior of these filters is provided in the NI-DAQmx Help (Start>>All Programs>>National Instruments>>NI-DAQ) in the book entitled "Digital Filtering Considerations for TIO-Based Devices".
I also wanted to comment on your original post and explain why you were receiving error -20020. For convenience, I have copied the text of the error code below.
Error -20020 occurred at an unidentified location
Possible reason(s):
Analysis: The cut-off frequency, fc, must meet: 0 <= fc <= fs/2.
I think you may have misunderstood exactly what the Filter express VI does. The Filter express VI takes a series of values in a waveform and performs filtering on those signals. So, it will look at a waveform made up of X and Y values and apply the defined filter to this waveform. Specifically in your application, the cut-off frequency (fc) is the Upper Cut-Off level that you specified in the Filter express VI; any frequency components of the waveform outside of the range you have defined will be filtered. The fs is the sample rate based on the data that you wire to the Signal input of the Filter express VI. In your VI, this data is coming from the DAQ Assistant. So, fs will be the sample rate of the DAQ Assistant and is associated with the rate at which points are acquired. The sample rate does NOT relate to the bandwidth of the signal, except that the Nyquist theorem tells us that the sample rate must be at least twice the signal bandwidth in order to determine periodicity of the signal. So, in this case, the sample rate of the DAQ Assistant would need to be at least double the high cut-off frequency.
However, you are performing a frequency measurement using a counter. I think this is where the confusion comes in. For the frequency measurement using a counter, the DAQ Assistant returns a decimal value which represents the rate of the pulse train being measured by the counter. It does not return the actual waveform that is being read by the counter. It is this waveform that would be band-pass filtered to eliminate frequency content outside of the filter's bandwidth. Instead of the Filter express VI, I would recommend that you simply discard values that are outside the range you specify. I have modified the code you posted earlier to perform this operation. The image below shows the changes that I made; rather than using the Filter express VI, I simply compare the frequency to the "Low Threshold" and "High Threshold". I use a Case structure to display the value on if it is within the limits. Otherwise, I display a "NaN" value. I have also attached the modified VI to this post.
Message Edited by Matt A on 09-04-2007 07:58 PM
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