adccommunitymod (AutomationDirect) asked a question.
Created Date: July 13,2007
Created By: jwbaker3
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We need to take 4-20ma analog input from 14 transducers and keep a 30 second FIFO stack for review if there is an error on the system. We also need to rebrodcast the info to Labview. The Desired sampling rate is 2000hz. What hardware will I need? If the sampling rate is greater than a PLC will take, what is the max rate? Is it possible to use two plc's and network them so one only works with the analog with a small program and the second can process the safety and operations to lower the scan time? The safety operations can sample at 100hz. Any suggestions are welcome I have to have 5 systems running by Sept. Thanks, JWB
Created Date: July 13,2007
Created by: jwbaker3
We need to take 4-20ma analog input from 14 transducers and keep a 30 second FIFO stack for review if there is an error on the system. We also need to rebrodcast the info to Labview. The Desired sampling rate is 2000hz. What hardware will I need? If the sampling rate is greater than a PLC will take, what is the max rate? Is it possible to use two plc's and network them so one only works with the analog with a small program and the second can process the safety and operations to lower the scan time? The safety operations can sample at 100hz. Any suggestions are welcome I have to have 5 systems running by Sept.
Thanks,
JWB
Created Date: July 13,2007
Created by: Steve Bailey
I don't think you 're going to get anywhere close to a 2000 Hz sampling rate with a PLC. The best you can hope for is an update once per PLC scan. There aren't many sub-millisecond scan PLCs available, and none that I know of from Automation Direct.
In addition, the analog modules use a successive approximation algorithm to convert the analog signal to digital and they don't refresh at that frequency.
Created Date: July 13,2007
Created by: franji1
I think the first issue is having fast enough I/O at the proper resolution.
What hardware is available that can give you 14 analog signals at (12 bit?) resolution at the rate of 2000hz? Regardless of PLC scan, you probably need to look at other technology with very fast analog I/O speeds.
You may need 14 independent A to D converters in order to get 2000hz - no multiplexing of A to D would work here.
Good, Fast, Cheap - pick two.
Opinions???
Created Date: July 13,2007
Created by: allukes
You say that you are using Labview. I 've never used it before, but don't they support some high end Analog cards? Could you do the number crunching in that and send whatever result you are looking for back to a PLC?
Created Date: July 13,2007
Created by: allukes
I just finished up a data acquisition project with a 260, which is one of or not the fastest CPU that AD has. I think the best you could do is acquire the data on every scan and even that is not good, because you need time for the signal to settle. I used Stage programming and could get a 3-4ms scan time, which would be 250 to 330 hz. Perhaps you could do a little better, but no where near the ballpark of 2khz. However, that is the least of your problems. The biggest issue is memory. Even at 250 hz, that would be 250 x 14= 3500 memory locations per sec. For 30 secs that's 102K of memory. If you want a faster rate and that many samples, I would suggest a PC based system.