How do I get a steady DC RMS value from a pwm signal?

[Enviro Engineer]

I have a motor controller circuit and Im trying to measure the ammount of switching going to a motor with my DAQ.  The signal I need to measure resembles a PWM signal but it varies in frequency instead of pulse width.  I would like to monitor the signal with Lab view.  Currently, Im using a DAQ 6009 to measure that PWM signal as an RMS voltage.  However this method is very inconsistant.  Can anyone help me with a way to monitor this pwm signal?  The idea is to be able to tell when the motor reaches a certain speed threshold.    

[Henrik_Volkers]

The DAQ 6009 has a timer, so why not measure the frequency? 
Use an analog LP with a high timeconstant?
Do some digital filtering with the sampled data, LP or median or ???


My crystal ball is in maintanance ....    so please provide more information about your needs.

How is your signal looking like? Spikes? Frequency range?
How fast do need to tell the crossing of the threshold?

[Enviro Engineer]

Proven Active Veteran Message 2 of 2 Viewed 4 times

Thank you for your fast response.  I am using the counter already for another part of this project, can I use it again?  I did not try using an analog LOW PASS FILTER or the digital filtering. My signal is like a square wave with a 95% duty cycle most of the time.  I want to detect when this wave changes to maybe 75% or lower.  But like I said the circuit is actually a PFM circuit because it only changes the On time.  This in turn changes the frequency, and unfortunately changes the sampling rate needed to get a consistant RMS voltage.  The checking speed can be relatively slow.  Im currently waiting up to 10 seconds to detect a change in voltage.

[S_Hong]

Hi Enviro Engineer,

Unfortunately you won't be able to do a frequency measurement task with your USB-6009. However, you may be able to simply acquire your data and use the Cycle Average & RMS VI which can be found in your Functions palette under Signal Processing » Wfm Measure. I believe that this may be the VI you are looking for as it essentially allows you to measure the average and RMS levels of a selected cycle of a periodic waveform or an array of periodic waveforms.

I hope this helps,

[Henrik_Volkers]

the pulse measurements.vi in your Functions palette under Signal Processing » Wfm Measure can might give you exactly what you want    read the help for the vi to see if it fit's your needs
might because my crystal ball is currently on calibration, so I don't know the PFM frequence range nor your sample rate

The analog filter is an R (10k -100k) from your PFM to AnalogIN of the DAQ and a C (100n-10µ? sorry no cristal ball) from that AnalogIN to ground.  Use it in case your samplerate is far to low.