Trigger aquisition across multiple compactDAQ chassis

[Jeremy Lea]

Hi,

I'm replacing two old DAQs which uses a central box with signal conditioning.  The systems are reconfigured and moved every few weeks, so the current sensors are on the end of long (20m+) cables to allow them to be placed wherever they are needed.  This causes a lot of noise...  The systems are outside in a wet/dusty/warm environment with a lot of moving machinery.  The channel count is relatively low (32 or so thermocouples per system and between 1 and 64 AI channels with a mix of AC and DC LVDTs, strain gauges, pressure cells, accelerometers, etc. - whatever the test needs).  The sampling rate is also low (max about 100Hz for the AI channels, triggered by machine location which is not under control of the system and has a non-uniform speed, much lower for the thermocouples), although I would like to move to taking multiple samples at each time (i.e. acquire 100 buffers per second and average the samples to get the final readings).

The current plan is to use a number of single module Ethernet CompactDAQ chassis (ENET-9163 or cDAQ-9181), each with an appropriate module and signal conditioning hardware, placed near a cluster of sensors, to reduce the cable runs on the sensors, and allow the system to be reconfigured more easily.  There would be a central chassis (cDAQ-9188 or cDAQ-9178) which would receive the signals used to start and stop acquisition and the master trigger/clock from the machine (at a macro level, not in the DAQ sense).  The plan was to use the PFI pins to distribute a trigger and clock to all of the modules to synchronize acquisition.  However, this has two problems: 1) it means running 4 cores of extra wire to each module, which is a pain, and 2) the new cDAQ-9181 modules don't expose the PFI pins (and you can't stick a DIO module in there too).

So I was trying to figure out the best way to synchronize acquisition across the various modules.  The timing needs to accurate to about 1ms or so (based on the numbers above).  Is this doable using internal routing in DAQmx? If I was to use a software trigger (i.e. just start acquisition of one buffer on each module without a trigger, based on a DIO acquisition on my central module), would that be fast enough?  Any other advice/ideas would be appreciated.

Regards,

  -Jeremy

[Paul-D]

Hello Jeremy, this is Paul with Applications Engineering at NI.

It sounds like you've done a lot of research on this topic.  I think based on the ruggedness your application require, that the cDAQ series would be the best to meet your needs and that the ethernet modules will help you achieve the distances that you require.  I will get into the use of the ENET-9163 later, but I would suggest you review pages 24-26 of the NI WLS/ENET-9163 User Guide and Specifications to ensure that your environmental conditions won't exceed the capabilities of the carrier.  Similarly, you may want to verify this information on the various modules you plan to purchase.

It's impossible to say if performing a "software" timing, aka starting all the tasks sequentially would produce the synchronization you are looking for.  Multiple things could affect the performance of this, and if nothing else, it will likely be inconsistent. 

There is a standard for synchronizing over ethernet, IEEE 1588, but unfortunately IEEE 1588 is not supported on NI CompactDAQ at this time.  That would have likely been the simplest way to achieve the synchronization you require without running extra wires.  Unfortunately the method of synchronization below is going to require you run extra wires, which is one of your concerns. 

As such you will have to perform some external synchronization as you've mentioned below.  The cDAQ-9181 will be out of the question because, as you stated, it does not provide externally accessible PFI lines and you can't use a digital module for sync in a single module carrier, because that would occupy the only available slot.  The ENET-9163 should meet your requirements for ethernet cDAQ and providing external PFI lines.

As far as synchronizing goes, the ability to route those signals varies module to module.  What Are the Allowable Triggers Routes on the WLS/ENET-9xxx Devices? offers a quick overview of the compatability of some of our cDAQ modules having their signals rerouted.  Additionally, if you would like to verify other modules not listed, you can do this by simulating the device in MAX, following the instructions in How Can I Know What Internal Routes are Available on My Device? and verify that the start trigger and sample clock are available for rerouting to PFI0 and PFI1.  As a general rule, most of our modules should be capable of this, but our delta-sigma Dynamic Signal Acquisition (DSA) modules will not support exporting the sample clock.

By exporting your Start Trigger and Sample Clock over these PFI lines, you should be able to achieve the 1ms synchronization you require.

Regards,