Magnetic Pick up Analog AC frequency measurement for Turbine Flow meter

[Aditya_K]

Hello!!

I am using the following hardware configuration to measure flow rate from a turbine flow meter.

Hardware: SCXI 1000 chassis

                  SCXI 1102 voltage input module

                  SCXI 1303 terminal connection block

Turbine Flow meter: output signal is ac frequency (30mv peak to peak). Sensor is magnetic pick up type.

I tried converting the output voltage signal to frequency domain using FFT but I am getting erroneous data.

I am wondering if anyone can suggest any good troubleshooting techniques so that I can figure out what is messing up my measurement or my measurement technique in labview itself is wrong completely?(From my understanding I believe I can measure the frequency of the ac voltage signal generated by the sensor by FFT. By measuring frequency I can calculate the flow rate)

I am aware that one potential cause might be ground loop in the system. The signal is grounded only in SCXI 1102 with factory default bias and pullup resistior network. I cannot think of any other reason for wrong readings or fault in my measurment. Also the flowmter is brand new and there is no chance of damage to the sensor.

I am sorry if I am missing out any important detail to answer as I am relatively new to data acquisition techniques.

Thank you

Regards

Aditya

[Henrik_Volkers]

 Some typical data to look at would be helpful 🙂

The easy way to get the frequency is the 'extract single tone detection.vi' , capture about 15 periodes of the signal ..

BTW: What frequency range do you expect to see?  If the frequency is above the line frequency and say 5 times the harmonics -> ~ 300Hz , you can use a software filter (highpass) or just tell the tone detection vi where to look for the frequency.

One trick you can try to minimise the ground loop ac hum:

Don't use the internal bias resistor, instead, connect a 100k potentiometer (trimmer) between A+ and A- and the wiper to AGND. Now, while looking at the measured signal, you can search for a hum minimum setting at the trimmer.

[Aditya_K]

Thanks for your reply.

I realized the problem is because the DAQ system (SCXI 1102) has a minimum voltage requirement of 100mV and my flow meter generates a signal with 30mV peak to peak. 

I was wondering if you have any suggestions on how to amplify the signal without altering the frequency of the signal. The frequency range is from 150Hz to 900Hz. This way I am thinking I can amplify the signal and then connect it to my DAQ.

I understand that this topic is not relevant to this forum but still I am hoping someone with good experience can guide me.

Thank you

Aditya

[Henrik_Volkers · Accepted solution]

It's about signal conditioning .... and thats what you want to do , so it's not so far away 😉

And if your signal has about 30mV peak to peak it fits perfect into the 200mV peak to peak range...  I would say no need for amplification ... the +- 100mV is not a signal minimum, it's the signal maximum in that range 😉

Hook it up, take a measurement in single channel mode with the a samplerate of about 10kSPS take 100ms (1000points)  and feed it into single tone detection .. 

display it with a signal graph , stop the vi, in the diagram rigth click on the graph icon and create a constant, save that in a vi and post it here if you need help in signal processing

[Aditya_K]

Thank you for helping me through the problem and educating me.

Appreciate your effort.

Regards

Aditya