configuring 1126/1327 in DAQmx

[LV_Pro]

Hi All,

I'm having some difficulty findin the right combination of settings, hardware and software, to configure an SCXI-1327 to work with a SCXI-1126 for making frequency measurements of a speed sensor signal. The sensor's signal is a high going low pulse, from 12V to 0V, 4 pulses per revolution, so it is normally high when at 0 RPM. The max RPM is about 6000, which gives us a max frequency of about 400Hz. I have not been able to find a combination of settings of threshold, hysteresis, low pass filter, etc., that gives me a reasonable reading. I'm also having difficulty with the 1327 in Max, where there are configuration settings for the attenuator. I have tried to set the physical attenuator to 1:1 in both the 1327 and its configuration in MAX, but everytime I go back into the configuration for the channel it is at the diffault setting of 100:1. We've tried to resolve this for a few days, without success, and need it running, so any suggestions are welcome.
LabVIEW 7.1.1, MAX 7.2.

Thanks,
Putnam Monroe

[Salvador_Santolucito]

Hi Putnam,

Are you seeing bad readings across your whole range or just at low frequencies? The range for the 1126 is 15 Hz to 128 kHz. So anything below 15 Hz will probably produce a bad reading.

The trick is to make all of your settings in MAX match what your jumper/switch settings are on the hardware. Are you creating a DAQmx Task in MAX and testing it? I think that that would be a good method to go through. There are also DAQmx shipping examples in LabVIEW for frequency measurements that you might want to take a look at.

In LabVIEW go to Help >> Find Examples >> Hardware Input and Output >> Analog Measurements >> Frequency

When you are setting up your SCXI system in MAX. Make sure that you are adding the chassis under DAQmx Devices. Also, you need to configure the 1327 as the accessory for the 1126.

You can find more info on settings by looking at the user manual for those devices:

http://digital.ni.com/manuals.nsf/websearch/1F9B70ADDC9BEA9F8625697A0069C745?OpenDocument&node=132100_US

http://digital.ni.com/manuals.nsf/websearch/1FF17019174F7CA086256CED005C957B?opendocument

-Sal

[LV_Pro]

Good Evening,
The set up has a number of other "modules" in the SCXI chassis. We are reading voltage, current and temperature on four units under test, and, for the most part getting good readings on those. I say for the most part because I was led to believe that the current and voltages were straight DC when in actuality they are pulsed DC (the system uses PWM). I've gotten by that with the addition of an RC integrator on each channel to give me the effective DC, dependent on the duty cycle of the pulses, but I can't find the magic numbers to set up the frequency measurements, and I have the question about configuring the 1327's attenuator in MAX. When I set it to 1:1 to match my the virtual channels' configuration and then go back in MAX to look at it the value has reverted to 100:1, so I don't know if it ever was 1:1. I'm also not sure what values of threshold, hysteresis, etc., would be appropriate for measuring a ~200 Hz signal that is a nice square wave, 12V going to 0V. As the motors will either be stopped or running at roughly full speed the band of interest of frequencies is probably in the 100 to 300 Hz range, so the 1126's lower limit of 15Hz is not a limiting factor. Our bad readings are across the frequency range, I haven't been successful getting it to properly trigger at any frequency.

WinXP Pro, LabVIEW 7.1.1, NI-Max 7.2


Putnam Monroe
Certified LabVIEW Developer
Senior Engineer
North Shore Technology, Inc.

[Salvador_Santolucito]

Hi,

We are now on DAQ 7.4 and I would like to suggest that you upgrade to the latest version.

http://digital.ni.com/softlib.nsf/websearch/90B60D5899BCCCDB86256FC700581B89?opendocument&node=132050_US

I do not see why your settings would change in MAX. Are you saving your setup in MAX after changing it?

-Sal

[LV_Pro]

Good Afternoon,
I don't see any method for saving in Max when I change the attenuators configuration. That isn't being done in the DAQmx area of Max, but is under the hardware configuration, under the SCXI chassis part.
I have MAX 7.4 downloaded, will try and find a way to get it on my customer's machine. I guess I will burn a couple of CD's as even the zipped down load is bigger than one CD. My main question is what would be valid configuration numbers (attenuator, low pass filter, threshold, etc.) for a 20-400Hz square wave with an amplitude of 0-12V P-P? I've tried a lot of combinations, nothing has come close to working.

Thanks and have a good weekend,

[Salvador_Santolucito]

Hi,

For the 1126 you should not need any attenuation. So in MAX and on the 1327 set the attenuation to 1.

I would suggest the 500 Hz range for your signal. Start with a threshold of 3 and adjust from there to see what works best. I do not think that you need to apply any filtering if your signal is clean.


Check out the specs for the 1126: http://www.ni.com/pdf/products/us/4scxisc306_ETC_196.pdf

User manual: http://www.ni.com/pdf/manuals/321844c.pdf

-Sal