cdaq 9188 PFI trigger voltage threshold

[Roy.a]

Hi,

I am using cDAQ 9188 with NI-9223, NI-9215,NI-9213 and NI-9375 modules respectively for data acquistion(3 different sampling rates). The data acquistion is in Finite Sampling mode and starts based on an external trigger signal connected to PFI0. In NI-9375 a DO channel is connected to a spark generator transformer relay which is based on another trigger from PFI1. PFI1 is connected to a pressure transducer mounted along the inlet flow path which measures the sudden rise in voltage when flow enters the system. Both triggers are based on Digital Edge, Rising. NI 9375, Vsup ~ 12V. 

Now, the situation is while decreasing the inlet supply pressure we observed that the PFI1 trigger doesn't work for a rise in voltage value (that comes from the pressure transducer) below ~1.9V. We checked in the 9188 manual that the positive going threshold for PFI channels is 1.43V.  Therefore, it seems like the trigger should execute up to a threshold of 1.43V. There are no issues with PFI0 trigger as the high on PFI0 is ~5V. 

We need to lower the supply pressure (less than 1.9V) for our experimental needs, but still would want to use this signal as a trigger. Any idea how we could achieve this (using PFI1 or any other sources as trigger - digital edge or analog edge etc.) ?

Thanks for your help!

Roy  

[Artem.SPb]

Roy,
Can you measure voltage in working circuit?
It is possible that impedance of wires and PFI making voltage lower than in anload output.

[yea_likethecity]

Hey Roy,

I'm with Artem, I'm thinking it's possible that the voltage rise from the pressure transducer may not be hitting the minimum threshold when you expect it to. I believe you are correct in thinking it should trigger once it passes the 1.43V threshold, though. Try checking the voltages at the module during run time and let us know if it's at 1.9 V at the module when the trigger activates. Also, how long does the voltage rise take?

[Roy.a]

Hi Artem and Austin,

Thanks for responding to my post and for the suggestions. I checked the imepdance for the PFI and it is ~81kOhms. Sorry, I am not expert with circuit and these numbers, so not sure what to infer from this. As far as I know the NI data Acquistion modules typically have impedances ~ or >1MOhm.

I did T off the PFI1 trigger to an analog input channel (NI 9223) and from that data only we figured out that the threshold was ~ 1.9V below which the DO module didn't activate as the trigger didn't excecute. 

The Voltage rise is pretty instantanous within fractions of milliseconds. I am sampling that data @ 250kHz.

Any other thoughts?

Regards,

Roy

[yea_likethecity]

Roy,

I've looked around online and found several resources which reference the positive going voltage and analog and digital logic so I'm going to link the most helpful article I found here. Check about halfway down the article, where it starts talking about analog signals.

Logic Signal Voltage Levels

http://www.allaboutcircuits.com/textbook/digital/chpt-3/logic-signal-voltage-levels/

Positive going thresholds generally refer to devices known as Schmitt triggers, and basically entail a threshold in which the trigger will activate, with some padding to prevent oscillation. As for your actual issue, I don't have a really good idea of what is going on right now but I'm going to ask around a bit and see if any of my colleagues have seen something like this. You may want to consider applying some amplification between the sensor and device that will boost the voltage from 1.4V to 1.9V so you can at least trigger on time.

[yea_likethecity]

Roy,

I've looked a little closer at your application and consulted some people on the floor and I noticed you're triggering a digital trigger with an analog signal. In this case, my previous note about Schmitt triggers is not relevant. With digital triggers, the minimum and maximum thresholds denote the points at which they are guaranteed to trigger. That is, a rising signal is guaranteed to activate at the maximum positive going threshold but may activate anywhere between the minimum and maximum. In your case, it activates at 1.9V. I'm going to link a white page which details techniques for triggering instruments, including one example of analog triggering. I hope it helps.

Tips and Techniques in Data Acquisition Triggering

http://www.ni.com/tutorial/4329/en/

[Roy.a]

Austin,

Thanks for your comments..

It was my understanding that for a digital signal high is always the highest voltage and low is the lowest voltage, therefore for there shouldn't be any Rising edge thershold, and even if there would be 1.43V mentioned in the manual. However, after your comment and going through the link on the book it seems like the rising edge can have any threshold.

I tried analog trigger with specifying a threshold voltage using the PFI channels, but it never worked. After some reading it seems like PFI channels are only for digital triggger. In that case, if I use an analog trigger what physical channel on the DAQ do I use? I know if we use any of the Analog input channels in the modules I have to change from Finite sample acquistion to Continuous sampling mode and also implement producer-consumer mode to deal with memory outflow.

Is there any way except for us to choose a lower range transducer, so that we can lower the flow rate and still get enough voltage output to trigger?

Thanks,

Roy 

[yea_likethecity]

This is the first thing I found, and seems relevant. 

NI-DAQmx: Analog Start Trigger to Continuously Acquire Analog Data

http://www.ni.com/example/29834/en/

I don't think it matters which channel you set the trigger for, only that you set the slope and level for the trigger correctly.

[Roy.a]

Austin..

Sorry for the really late response, just to let you know that we finally used a lower range transducer and adjusted the output scale so that the trigger voltage was always ~5V. That way we could get consistent trigger for the entire range of flow rates (lower pressures) we wanted..

Thanks again for your help and the links you posted was certainly helpful to gain more knowledge on how these circuits work..appreciate it..

Roy