[Dsg-hallb_magnets] [Dsg-internal] cernox sensors.

[Amrit Yegneswaran]

thanks for the info brian. 
glad to know you are back. 

since it is algorithmic on the spare excitation chassis, one could infer that the spare excitation chassis is in the clear. 
for the one in use, one can't make this inference, i guess because you haven't tested it. 

one other question: since you say the resistance goes down to 0 ohms, V1 must be very very close to V2. 
so when the difference is taken, could there be any precision/ bit resolution/ rounding effects in play. 
i'm sure you have looked at this possibility. 

amrit 




From: "Brian Eng" <beng at jlab.org> 
To: "dsg-internal" <dsg-internal at jlab.org> 
Sent: Thursday, February 9, 2017 12:45:04 PM 
Subject: Re: [Dsg-internal] [Dsg-hallb_magnets] cernox sensors. 

The current plan in place is to restart the cRIO, the new one should be to run though the startup sequence for the cernox sensors if the value is bad. 

It generally gets stuck at 325 due to reading the same voltage twice, so when it does the calculation for resistance (v1-v2)/(i1-i2) it gets 0 ohms, which from the lookup table is coerced to 325. 

I was never able to get the spare excitation chassis to do that once I changed the algorithm logic (I could get it stuck running Christiana's original algorithm), so as to why it gets those values, I'd suspect something with the excitation chassis since all the cRIO is doing is telling it to set the current and reading back the voltage, but since I can't duplicate the problem I can't say with any certainty. 

> On Feb 9, 2017, at 12:28 PM, Amrit Yegneswaran <yeg at jlab.org> wrote: 
> 
> i understand that there is a plan of action if the cernox sensors read 325K. 
> restart. 
> 
> but does anyone understand why it reads 325 K? 
> is this a feature or a bug? 
> i guess there is look up table and when the resistance goes below a certain value the output is 325 K 
> so the academic interest is to understand whether it is a issue with the look up table or the hardware, or the software. 
> brian, peter, and tyler, perhaps you could try and explain to me your understanding of the matter. 
> 
> amrit 

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