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Thread: Adapting Teensy for Industrial Automation(Arduino Compatible Shield,Intrinsic Safety)

  1. #1
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    Adapting Teensy for Industrial Automation(Arduino Compatible Shield,Intrinsic Safety)

    I am working on developing IoT sensors that can be deployed in Industrial environments - especially ones that are battery powered, wireless(mesh network), and are intrinsically safe(https://en.wikipedia.org/wiki/Intrinsic_safety).

    Currently I have been experimenting with Teensy 3.6 and like what we see with the main board and the shield. I used Digi XBee Pro for wireless mesh network. I have run into some practical challenges with productizing this further.
    I would love to get some inputs on how we can adapt Teensy to the Industrial IoT area - especially:

    1. How to get an Arduino compatible shield designed for Teensy 3.6 + Prop Shield. The idea is that with this type of shield, we can tap into arduino shield ecosystem for additional sensors and I/O capabilities like 4-20 mA/RS485 etc. We tried using the teensy shield from sparkfun, but it was only 3.1 compatible and it is not arduino compatible.
    2. How to get an intrinsically safe board designed for Teensy. Its not that complicated based on what I have read, most of it is some changes to power flows(if at all needed) and then getting testing/certifications.

    Any suggestions/inputs welcome.
    If anyone is able to offer this in return for compensation, I am willing to consider.

    Thanks.

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    Regarding intrinsic safety, I've only been indirectly exposed through my job in which we certified for a complete, portable battery powered piece of equipment; not a component/sub-assembly.
    Regardless, I thought I'd comment as this is a pretty niche thing so I'm not sure there will be others on this forum with more experience. Maybe something I say is useful.

    Do you have an idea on a few of these points?
    * which regions are you interested in (USA, Canada, EU, etc)?
    * what level of safety and environments is your equipment to be used in (e.g. near open areas with gasses)?
    * what ambient operating temperatures will your equipment need to work in?
    * how will it be powered?
    It's worth seeing if there are similar products - or products you'd be connecting into - and see what certification ratings they have and what 'entity parameters' they have (if that's available).

    From my experience, if you can design for IECEx/ATEX, you're designing for the toughest.
    Pretty early on, you may have to invest in get a hold and reading some of the applicable standards (e.g. IEC 60079-0). Probably worth seeing if you can get some advice from a consultant to get you started on the right path.

    Practically, a lot of it seems to come down to limiting energy and working within specified tolerances and safety margins
    - capacitance and inductance are the enemy. You make your life easier if these aren't over done. Series resistance can also be your friend.
    - track clearance and creepage is important. Higher IP ratings / encapsulation help
    - temperature rise in fault conditions is critical. For battery powered systems they do things like short circuit the battery.
    - barrier circuitry is important. Depending on the ratings you go for, this can impact how you layout redundant componentry (you'll see terms like 'countable faults', 'infallibility').

    - you'll end up having 'critical components' in your design so be careful of not pushing yourself into a corner for manufacturability - these components can't just be swapped out.
    - you may have to deal with periodic auditing if you are considered the manufacturer/assembler. This may not be initially apparent but it can make up quite a bit of the cost in manufacturing a product if you are thinking small scale production.

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    Many thanks for the helpful tips LachAus.

    Let me clarify few things:
    The intent is to build a battery powered, wireless IoT device that can sense, do some processing and send out data. Example measure lateral displacement of Tank roof using ultrasonic/laser range finder. There are many more use cases. The use cases need Zone 0/Zone 1/Zone 2 approval in Gas Group IIC with temperature rating T4(135 deg C). For temperature, even T6(80 deg C) would work.
    Region - Middle-East, Asia, EU.

    I agree designing for IECEx/ATEX makes most sense as its extensive and well adopted. I have skimmed through IEC 60079-0/1/11 though I confess I do not understand it fully because of the way its worded(not because of technical complexity), but understood at high level what the standards are looking for. However my challenge is how to get the gist of IEC 60079-11 in a form that is understood by someone who designs circuits.

    What you have listed for designing circuits is very helpful...I wish there was a more technical interpretation of IEC 60079-11 so you can verify various clauses against your circuit design.

    Would appreciate if you can share any experiences around testing/certification process as well. What works/doesn't work?

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    This is not the forum for IS design. Your team needs to invest in an initial study of the software and hardware and environmental standards that are scoped for your category and class.

    One of your team should be designated to do the the compliance stuff (safety/EMC/environmental/life cycle. Send your designated victim/engineer to a monthly meeting of a PSES chapter (several local chapters of the IEEE PSES are up and down the California coast) to ask some specific questions on IS/ATEX. They will be able to point you to a specific person at a local NRTL whom does this stuff.

    Cold-calling your local NRTL will probably result and wading through multiple sales dweeb pitches - so you need a name prior to calling.

    If your team has no experience with the basic regulatory machinations and physics of an LPS, then IS will be a tough nut to crack without external help.

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    Looks like what you're going for is actually a little more similar to my experiences than I initial thought. 60079-0 and 60079-11 where applicable. Still, I'm just a firmware guy so don't put too much weight on what I have to say.

    I've definitely seen the standards been left to peoples' interpretations in places. It can be a little tricky to know exactly what needs to be done and what can be argued for.
    I can't give much first hand experience on the process itself. My feel is - like other certification process - some of the test houses are willing to work with you a bit (more likely if you're in the timezone) but some can be painful. Being helpful and try to do some due-diligence seems to go a long way (e.g. pre-calculate capacitance, get critical drawings prep'ed, get your manuals in order, etc).

    Some more of the practical hardware side of things that come to me:
    Power
    * Are you going primary or secondary (rechargable)? - There are limitations on the chemistry allowed in the standards depending which way you go. I'll assume 'secondary' from here (my experience has been with rechargeable lithium ion)
    * you'll need to have some protection like a fuse and diodes and maybe some small resistors between the battery and power to the rest of the circuit. Be careful here with the practical implications - you'll get voltage drop as your circuit draws current if you end up with lots of resistance. Also, be weary with your feedback path to your charging IC as some of those components may mess up it's ability to sense charge complete.
    * They will perform a short circuit on the battery. I think if you only go 'ib', you may be able to leave some protection circuitry in if it meets certain specs. if you go 'ia', I'm pretty sure you have to remove all the protection circuitry, including any battery PCM. May be worth looking for cells that have some extra protection features built-in (such as PTC) as I *think* these are valid in the 'ia' short circuit test.
    * Batteries tend to be the limiting factory I've seen in temperature rating. I don't think you'd be able to get better than T4. Pushing above Ta > 40 degrees C apparently starts getting difficult. For ATEX/IECEx they will do the short circuit test in the maximum Ta temperature specified. Encapsulating the battery may help reduce the surface temperature and may let you achieve a better Ta. Also, if the cell is enclosed and leaks but your encapsulation keeps it contained, I think you can avoid a fail.
    * We had to add some mains AC voltage protection to the charging input. Though this not intended to be used in a hazardous area, it's part of the newer revisions of the standard I believe. A fuse and a crowbar circuit are probably the way to go.

    IIC:
    * all the calculations have a safety factor. I think IIC factor is x1.5
    * careful of total capacitance. There are tables in the standard which define the maximum allowed. I think they stop at 10V. If you're below that system voltage but work within the 10V constraints, you won't have to do additional testing.
    * may help having an enclosure with some conductivity (less than 1 giga-ohms I think is the spec).
    * enclosure IP rating (IP64?) will help you with trace spacing and maybe some other requirements

    Basically every I/O that will connect to something external needs barrier protection. Your protection rating ('ia', 'ib', etc) will indicate how this needs to be done for infallibility / countable faults. Some examples:
    * DC block lines with 3x series capacitors
    * resistance + zeners (limit current and voltage)
    * Typically a DC powerline will have a fuse + 2/3 series diodes
    Specs on these components can matter (e.g. capacitor voltage ratings). Using pre-qualified component like a fuses (e.g. see Littlefuse) may be useful.

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    BJB's advice is probably the best so far in this thread

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    BJB - thanks for the tip on NRTL/IEEE. At this time I have fair familiarity with IEC 60079-0/1/11 standards that I understand at a higher level what is being checked for, but need tips on actual circuit design. I have talked to UL team and their knowledge is about mine level on standards - nothing on circuit design. Do you know if NRTL/PSES visits can benefit the design of circuits or just understanding standards ?
    I understand this forum is not for IS design, but since this is for new project guidance I thought worthwhile to ask for tips on the subject from more experienced members. If you have link to IS design discussion forum, I will appreciate it.

    LachAus - many thanks for listing the details on circuit design and tips on testing prep. This will be an incredible help with interpreting the tedious standard. It has also helped confirm my limited understanding of the IEC standards. I am definitely going to save these for my final checklist. I will also do another reading of the standards hoping more of it sinks in I have seen these industrial sites first hand and seen many bulky/heavily powered apparatus so can see how this design is all about the energy storage and dissipation under faulty conditions, rather than how much energy goes in.

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    Typically, NRTLs will not, and cannot, provide design services, other than to review a design proposal or do a gap analysis on an existing design. If your knowledge of a standard exceeds that of an NRTL assessment engineer, then talk to someone else or another NRTL. For UL, an excellent source is the PDE for that particular CCN.

    There is nothing special about circuit design for this stuff. It is all about controlling power and energy. Look at the Type Tests for the scoped standards and design a box that will pass these tests.

    Start with simple standards for reliable limited power sources where the physics are obvious, such as IEC62368-1, UL1310, UL5085-3, etc. Next look at enclosures, cables, connectors, and other materials intended for use where gas and dust are problems. Then look at the standards for these materials such as UL1203 and UL913 UL698.

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