[ RadSafe ] Interlock question

[jjshonka at shonka.com]

Fermilab had an interesting interlock system which replaced the micro switches that Ted might prefer and instead used two pairs of coupled photodiodes and photo detectors embedded in the hinge.  When the hinge was closed, the diodes and detectors were aligned.  They were run with square waves (I think) at a different frequency than AC (60Hz in US) and 90 degrees out of phase so that the door was checked at twice the frequency.  This is much better than micro switches which can be shorted out and appear to be closed, and when properly operating only indicate closed unlike the Fermi system which was interrogated many times per second.


Joe Shonka






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From: Ted de Castro
Sent: ‎Thursday‎, ‎October‎ ‎30‎, ‎2014 ‎4‎:‎23‎ ‎PM
To: The International Radiation Protection (Health Physics) Mailing List





Thanks Bob - I'll look that over.  Yet my question is still with regards 
to more primitive electronics - discrete logic, gates, buffers etc. be 
it TTL or CMOS or whatever.  These are vulnerable yet not as engineered 
and thought out as a safety certified PLC - but certainly less 
complicated and don't involve software.

I don't see a compelling need to employ such in interlock circuits and 
thus prefer to stay with the tried and true, and easy to analyze and 
test, switches and relays.  It doesn't matter if failure is more 
frequent - if by design that failure is failsafe - and better yet 
detectable.

Like I said - I couldn't get consensus on that from the writing 
committee on ANSI N43.2.  Its hard to say why.

ted




On 10/30/2014 11:21 AM, Bob May wrote:
> Excellent discussion on interlock design and functionality. There is a Rockwell publication at the following link: http://discover.rockwellautomation.com/Files/PLC-vs-Safety-PLC-Fundamental-and-Significant-Differences.pdf that discusses PLCs versus Safety PLCs. It won't change your mind if you are not a fan of electronic systems but it is informative and points to the international standard IEC 61508, "Functional safety of electrical/electronic/programmable electronic safety-related systems".
> Bob
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