Voting 2oo2
Voting 2oo2
(OP)
Hi,
We have 2 sensors and we are planning on wiring them so that both have to reach there setpoint before a controller action takes place.
I have been reading that the reliability will be improved because spurious tripping will be reduced but on the other hand the failure rate has now doubled as if one sensor fails the controller action won’t take place.
What is the general consensus on using 2oo2 voting on machinery, is this something that is normally done ?
Thanks
We have 2 sensors and we are planning on wiring them so that both have to reach there setpoint before a controller action takes place.
I have been reading that the reliability will be improved because spurious tripping will be reduced but on the other hand the failure rate has now doubled as if one sensor fails the controller action won’t take place.
What is the general consensus on using 2oo2 voting on machinery, is this something that is normally done ?
Thanks
RE: Voting 2oo2
When using 1oo2 logic, failure rate is effectively doubled because failure of ANY of the sensors will cause spurious trip. You have two sensors with failure rate 0.01 hence the system failure rate is 2x 0.1 = 0.02
When using 2oo2 logic, you need both sensors to fail simultaneously, i.e. 0.01 x 0.01 = 0.0001 hence the system failure rate is effectively reduced 100 times.
Dejan IVANOVIC
Process Engineer, MSChE
RE: Voting 2oo2
Thanks for this. I just have a question. If the sensors are “ANDED” together and one fails to read the desired setpoint then would this not prevent the other sensor from working correctly as would this not feed into the second sensor ?
Thanks
RE: Voting 2oo2
IF (NOT (Fail_A OR Fail_B)), then Trip = Trip_A AND Trip_B
IF (Fail_A OR Fail_B), then Trip = ((Trip_A AND (NOT Fail_A)) OR (Trip_B AND (NOT Fail_B)))
Now, your actual hardware failure rate is doubled, but that's a cost/maintenance time issue.
TTFN (ta ta for now)
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RE: Voting 2oo2
Dejan IVANOVIC
Process Engineer, MSChE
RE: Voting 2oo2
Both sensors will be taken back to relays in a panel and the second sensor will only be initiated if the first has detected water, so if there is a fault in the first then the second sensor would never be initiated to look for water and the pumps would never trip.
Thanks
RE: Voting 2oo2
If S2 = Trip, AND S1 <> Trip, Pull Plug (anyway) Check S1 and room
Seems to me that you are actually trying to prevent inaction on the operator's part, so why is there even a human in the loop? All the testing and actual usage on autonomous cars and other systems show that the human is the weakest link and least able to sustain alertness for more than about 10 to 15 minutes. If the plug is automated, then you can apply normal reliability and redundancy calculations to crank up the probability of missed leaks to as much as you want. If the human is in the loop, then even a 99.99999% consistent trip can still be missed by a texting operator.
TTFN (ta ta for now)
I can do absolutely anything. I'm an expert! https://www.youtube.com/watch?v=BKorP55Aqvg
FAQ731-376: Eng-Tips.com Forum Policies forum1529: Translation Assistance for Engineers Entire Forum list http://www.eng-tips.com/forumlist.cfm