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SCXI-1140

Hi,

 

As a part of my setup for the friction measurement of incylinder pressures, I will be using an SCXI-1140 simultaneous sampling with an SCXI-1301 connector block, mounted in an SCXI-1000 chassis.

 

I'm currently getting used to the system to try and acquire the data according to an external sample clock (TTL). Is it possible with the SCXI-1140 ?

 

Best Regards

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Hi BCar,

 

What DAQ device are you using to digitize your SCXI-1140 signal? The ability to use an external sample clock will depend on your DAQ device's ability to receive the clock through a PFI line. One issue I forsee is that if you only have one connector block on your DAQ device (which will be used by your SCXI-1140), then you won't be able to access any of the PFI pins. In this case, you would have to have a DAQ with dual connectors, or have some sort of breakout board to gain access to those pins.

Mark M.
Applications Engineering
National Instruments
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Ok then thanks Mark. Another issue which is confusing me is the 'hold' terminal on the NI-1140. What is it for exactly ? It seems to output a square wave when I run my LabVIEW VI.

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Hi Mark,

 

The Data Acquisition board I'm using is a PCI-6251, together with LabVIEW 11. I noticed that the hold terminal on the SCXI-1140 corresponds to PFI7 on the PCI-6251 through a 200Ohm resistance. In fact I can use the PFI7 through the hold terminal as a sample clock. However I'm not really sure that the signals are sampled simultaneously. To check this I'm using a sine wave from a signal generator. The output from the signal generator is input to three channels (differential) on the SCXI-1140. The signals are sampled with a TTL signal on the Hold terminal.

 

It seems that the three sine waves are acquired with different amplitudes, depending on which channel on the SCXI1140 they are input. Any help ?

 

Thanks for your time.

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Hi BCar,

 

Can you simultaneously plot the three channels on a chart and post a screenshot of that? Also can you attach your code?

Mark M.
Applications Engineering
National Instruments
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Dear Mark,

 

Attached are two word documents with screenshots of the three traces plotted simultaneously. In the screenshots I'm zooming closely on the trough of the sine wave being acquired. One .doc file shows the screenshots when data was sampled using the internal sample clock, and the other .doc file shows screenshots when data was being acquired using an external sample clock on the hold terminal (TTL signal). All the traces were acquired in Parallel Mode (not multiplexed).

 

In the setup I'm using the SCXI-1140 mounted in the SCXI-1000 chassis (with no other attachments). The DAQ I'm using is a PCI-6251.

 

The setup is coupled with LabVIEW 2011 and to keep things simple I'm using DAQ assistant as a VI.

 

Thanks in advance

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Hi BCar,

 

I will take a look at what you have attached and do some research. I'm sure that I have some colleagues here in Applications Engineering that will be able to provide some insight into this, too. I'll get back to you as soon as I can!

Mark M.
Applications Engineering
National Instruments
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Thanks Mark 🙂

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So my guess at what is happening is the HOLDTRIG signal that you are using as the clock signal has a large skew from when the sample is actually taken on the SCXI module. Keeping samples in the sample-and-hold circuitry on the module causes "droop" where the amplitude of your signal can decrease the longer it is held. Ideally, the HOLDTRIG signal probably shouldn't be used as an external clock source. The only two options I can see for your application are to either get a PCI DAQ with dual connectors, or modify your cable so that you have a free wire to a PFI line.

Mark M.
Applications Engineering
National Instruments
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Hi Mark,

 

Sorry for the late reply. I tried using the PFI7 instead of the 'Hold' terminal on the SCXI-1140, however the problem still persisted at fast sampling rates. I'm perceiving the problem to be related to the sampling rate. I'm using three channels (differential) at 3000RPM with an incremental encoder of 3600ppr. This means that sampling would be at 180kHz.

 

Looking at the SCXI-1140 manual (screenshot attached) it states that for the minimum settling time (0.012%), the acquisition takes 7 microseconds. On the other hand if the 180kHz (sampling rate) is converted to periodic time, it means that at 3000RPM, only 5.5 microseconds are allowed for acquisition to happen.

 

What are your opinions on this reasoning ?

 

Best Regards,

Carl

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