cDAQ or cRIO or RT+cRIO?

[1984]

Hi Community,

I have to write an software which controls a process can last up to 8-10 hours. This is a high current application so the stability of the system is a fundmental requirement. I need to choose the right hardware for the appication and as I'd like to go with the c-series modules I am hesitating between going with cDAQ, cRIO or an RT+cRIO (eg. 9081).

I am somewhat concerned about using cDAQ as it depends on Windows, so if Windows hangs my application hangs as well which is quite undesired. In the other hand I was able to write appiciations using cDAQ which can run for weeks without any problems (obviously automatic updates are disabled on windows and we have a UPS in the system). The timing of the inputs / outputs certainly dont require the timing accuracy of an FPGA based device)

Which would be my best option amongst these three for a station like this?

thanks.

[timtamslam]

What kind of process are you controlling and what kind of rates are you looking at for control? Are you at all concerned with your controllers ability to deterministically apply command values to the system?

[1984]

I am controlling high current rotating machines with the app. Sampling rate probably not need to go over 5kS/s and the control signal timing doesnt require us timing.

What I am mainly concerned about that what if the host PC starts hanging. In this case I guess a cDAQ unit will hang as well which means the UUT may end up in smoke / fire.

[timtamslam]

I just wanted to make sure the control speed requirement wasn't overlooked as there is a fair amount of latency when setting up control over USB. 

Your concern is very valid. In the circumstance where you are closing the loop in Windows from the cDAQ controller, if for any reason the Windows system goes down (or the cable is disconnected), the output modules would continue to output whatever values they were last configured for. That can certainly be a scary situation for your hardware and cRIOs have a nicer coupling to the IO which can be advantageous in these types of situations. 

However, I believe you can avoid this behavior and still use cDAQ by configuring a hardware watchdog in DAQmx. In the event that he applicaton hangs or the cable is diconnected the reset timer wouldn't reach the hardware, the watchdog would then run late, and the IO would move into a safe state that you have configured. 

How to use and implement the watchdog timer with DAQmx

http://www.ni.com/white-paper/14616/en/

Based on what you've described and your concerns I think cDAQ could work for you so long as you implement a watchdog. 

[1984]

Thanks, it looks very promissing. I mostly use NI 9174 (USB) and NI 9178 (USB) chassis, and it seems watchdog is not supported on those. Is this really the case? What would be my closest alternative if so?

[timtamslam]

Ah, bummer. The only thing that comes to mind next is setting up an exeternal circuit to act like a watchdog that's connected to the cDAQ chassis power... basically it will listen for a heartbeat digital signal you send out from a module, and if it stops getting the heartbeat, it will switch power off from the controller. That approach definitely has its downsides though  Aside from that, an embedded controller approach (like cRIO) is what comes to mind. 

Someone else may have some more experience in this type of application though. 

[1984]

I think the NI 9188XT can be an option for us. Can you confirm this? (I just google an external watchdox device, that was my idea as well at the beginning but I guess it increases complexity and the overall price is gonna be the same as if I just buy a cDAQ chassis with watchdog)

[timtamslam · Accepted solution]

I believe it is, yes. I have seen it mentioned in a couple different documents and we even have a video demo of the watchdog functionality for the 9188XT. 

NI cDAQ-9188XT - Product Page

http://sine.ni.com/nips/cds/view/p/lang/en/nid/211734

• "Configure a fail-safe mechanism using the watchdog timer for output types to react appropriately"

NI ComapctDAQ Watchdog - NIWeek 2013

https://www.youtube.com/watch?v=Occkq1Ax3DI

Definitely a good option for this type of situation