Using a 9205 to measure RPM

[tkopinetz]

I have not used labview in over 10 years but now have an application that requires but I am more then rusty. I am trying to measure RPM using a 9205 module and I am sure there is a way to do it. I have a pulsed voltage output from my measurement circuit and so far I am capturing the voltage. Can someone suggest a way to convert these voltage pulses to an RPM value. I am sure some type of leading edge counter would work.

Thanks

[Broken_Arrow]

You may be capturing a voltage, but you have no relation to frequency (the pulses), so there's no "RPM data" in the voltage you are measuring.

Have you considered a frequency to voltage converter? That'll give you a relationship to RPM that the all-Analog 9205 can make sense of.

[Hooovahh]

Not the cheapest route but there is a SCXI card that measures frquency.

http://sine.ni.com/nips/cds/view/p/lang/en/nid/1661

There may be some other frequency cards this is one I have used in the past.  With this card you can create a virtual channel and ask it what frequency the analog signal is going into it.  From here there is a conversion from frequency to RPM depending on the type of resolver or hall signal you are using to generate that signal.

This method is the easiest but the cheapest would be to get a cheap AI DAQ device that can measure several times faster than the generated signal (nyquist theorem) then analyze the series of AI reads to determine the frequency.  Then convert frequency to RPM.  Depending on the signal a USB DAQ device might work, if not a PCI or PXI + MXI card would still probably be cheaper than the SCXI route.

[John_P1]

Are the pulses TTL compatible?

If so, the 9205 actually has a PFI line that can be used to route ttl signals to the chassis backplane.  Assuming you're on a Compact DAQ chassis, you can configure a frequency measurement using the counters on the backplane as follows:

Compact RIO would of course be programmed differently.  If you are on one of the single-slot carriers (916x) there are no counters available so the above method wouldn't work.

If the signal isn't TTL compatible then the above also would not work.  In this case, you could always process the acquired analog signal to determine the frequency.  For example, with a known sample rate and voltage threshhold, you could check the acquired array for the number of samples that occur between threshholds to find the period of your pulses.  Invert this to give frequency in Hz.

Best Regards,

[tkopinetz]

What I have is an analog signal from a pulse converter being used onboard to measure the RPM of a fan to control the circuit. I was hoping to be able to count the pulses using the leading edge function on the 9205 module. My voltage starts at about 1 volt and when pulsed will be around 1.5 to 2.5 volts. I did consider building a quick circuit to convert the analog pulses to digitial and then using a digital I/O module to capture the count.

[John_P1]

The 9205 actually has analog triggering capabilities.  When you configure an analog trigger, the Analog Comparison Event can be used as a TTL signal that is derived from your analog signal.  What chassis/sleeve are you using?

Best Regards,

[VladimirK]

I'm trying to use a 9205 card for a similar application (RPM meter). I have a cRIO 9074 and I'm trying to get an analog trigger to go off when the voltage rises above a certain value, and activate the digital out, which I then connect to a counter (another module in the cRIO) . So far I haven't been able to make the analog triggering work. I'd be glad if anyone can advise me as to the considerations when using analog triggering.

Vladimir.

[Patricia B]

Vladimir,

Are you planning you implementing this on the FPGA or real time?

Regards,

[VladimirK]

I intend to implement this on the FPGA, since I'm measuring a signal with max frequency of around 14kHz, I figure I need to sample at least at 100kHz in order not to miss anything.

I've actually managed to get the trigger working now, using the "set triggers" method node, so that the trigger goes "true" each time the signal is above a certain level (with hysteresis). I tried to wire it to the digital out of the 9205, which I wanted to connect (physically) to a digital input module I have running as a counter in scan mode, but the digital out doesn't seem to work, at least I get nothing looking at it with a scope.