Best method for synchronizing two cRIO chassis populated with 9233

[Vahan]

I have two absolutely identical 8-slot cRIO systems (9104/9002) each populated with seven 9233 and one digital input modules. Data from both cRIO controllers are transferred to host PC via TCP IP at 2kHz per channel. I need to make sure the data from both chassis are synchronized. I figured out how to synchronize all modules in one chassi and it worked quite well. However, two-chassi synchronization is more challenging. The multi-chassis synchronization example is not working properly for 9233 modules (too much drift and some data loss). I'm guessing this is due to the fact that 9233 is a self timed module, so you can't just trigger the loop with DI. There has to be another method for multi-chassis synchronization for DSA modules. Any idea has this can be done? Thanks.  

[Jared_B]

vahan,

Currently synchronizing multiple cRIO chassis with DSA modules is not supported.  You are correct regarding the self-timed internal clocks of the DSA modules, this is where the issue comes from with the synchronization between chassis.  This is possible in cDAQ chassis, but not currently supported in the cRIO chassis.

[Vahan]

Jared,

Thanks for the info. Is NI planning to make such synchronization possible for cRIO platform any time soon?

Vahan 

[preston_johnson]

Thanks for the post Vahan,

There are a couple of methods using re-sampling from our Sound and Vibration Measurement Suite to synchronize the measurements.  In particular, we need to share some clock - say a 10khz or 20khz pulse between the two digital modules.  Then digitizing these along with the vibration signals provides a common reference with which we can re-align the data.  The sound and vibration even-angle resamplinig functions can use this pulsing clock to align the data.

My team can provide a proof of concept or talk with you in more detail. 

[Vahan]

Preston,

Thank you for your quick reply. I have a GPS PPS at 1Hz available for both digital modules from individual GPS reveivers. Is this 1Hz GPS pulse (10% duty cycle, less than 100ns accuracy between two receivers) going to be sufficient for the method you described? Do you have an example project that I can use as a reference?

Best regards,

Vahan 

[Jack_A.]

Are your cRIO chassis going to be located next to each other or far apart?  What do you plan to use the GPS PPS signal for? 

The reason I ask is that I have some code which will synchronize multiple cRIO chassis with delta-sigma based modules such as the 9233.  It's not completely polished, but it's very functional.  This code uses one of the DIO modules to generate a "reference clock" at a certain frequency, both chassis import this clock and then resample the data in teh FPGA to this new timebase.  This works great if a cable can be share between the chassis.

Just curious, but what's your application?  What are you going to do with the data once you've acquired it?  Do you have a phase mismatch or synchronization requirement?

-Jack

[Vahan]

Jack,

The units are located far apart. One module is at the base of large wind turbine, the second one is on the wind turbine rotor rotating at about 20-40 rpm.  The rotating unit has a wireless Ethernet access point attached to it, so it sends packets of data via TCP/IP to the host PC. I need tight phase match (less than 1milisecond) between both chassis since I'm measuring vibrations, strains and position for turbine blades and other structural components.

I have been trying to send a clock signal to the rotating module through slip rings, but it did not work quite well because of high frequency noise present in turbine power electronic components. So, I have purchased GPS+ modules from S.E.A. with antennas, installed them in slot 1 of both chassis and started experimenting with synchronization via GPS. Everything works fine and stable (stable GPS signal and solid wirless Ethernet connection) except for synchronization. I get slow drift between time series from both chassis over time. If you have time to look at it, I can send you a copy of my project (to your E-mail address, I'm not allowed to post it on the forum).

You think the code you mentioned in your post could work with GPS as well? The GPS+ modules generate very precise pulses that can be accesed from backplane. 

 Using the GPS for synch would be the best solution for me, because as I mentioned, the 9401 DI modules get confused because of noise in sliprings. Besides, high quality sliprings are expensive and hard to install, so I'd rather use GPS.

[Jack_A.]

Yes, GPS is definitely the way that you'll want to go.  I have some ideas for you and would love to take a look at the code you currently have.  My email is jack.arnold@ni.com.

-Jack