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Thread: PLC

  1. #1
    Join Date
    May 2007
    Location
    Michigan
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    319

    Hmm PLC

    Heya Guys
    Question
    How come you dont see more PLC's in buildings?
    I understand price is about triple to have a system like this in there but from what little I know and even less than Ive read these things are like cockroaches - they never die?
    One guy put it best - He asked have you ever seen a HVAC Controller 2 miles beneath the earth in a extremelly humid and dusty environment.
    The answer is no - he goes on to explain all of the things in a coal mine PLC's control. Other than a Siemens system at the mall I have never seen a PLC used in HVACR. Everything was HVACR specific?
    Has anyone ever been the architect behind the design for one of these systems?

  2. #2
    Join Date
    Aug 2009
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    2,459
    Equipment for coal mines has to be 100% spark proof. The types of relays we use above ground could cause an explosion down in a mine. That is why every piece of electronics in a mine is specially designed and specially certified.

  3. #3
    Join Date
    May 2007
    Location
    Michigan
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    BACnet

    Heya
    This dude was just comparing robustness.
    Actually one of the points of robustness.

  4. #4
    Join Date
    Aug 2009
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    2,459
    People post here all the time about HVAC systems put in place 25+ or more years ago that are still chugging along just fine. There is big business in gateways to speak to those older systems.

    Reading all of those posts I don't get the feeling that there is any categorical lack of "robustness" in the HVAC industry.

    But I would prefer to use an HVAC controller to a PLC when doing HVAC work because they were designed to make HVAC work easy. They have communication lines to allow them to communicate over the net, they have applications to control common HVAC equipment, they have control loops that are easy to tune for the time constants commonly found in HVAC applications, etc.

    That PLC-based mall system you alluded to will be a real bastard when they renovate or build on to the buildings. On the other hand, if they had chosen a proper system (Lon or a BACnet) adding to or modifying it will be a piece of cake.

  5. #5
    Join Date
    Jan 2011
    Location
    London, England
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    PLC: Programmable Logic Control isn't often used in buildings as it isn't very well suited to analogue applications such as temperature, humidity and pressure.

  6. #6
    Join Date
    May 2007
    Location
    Michigan
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    Thumbs up O

    So I see.
    The mall system is somewhat 7 years old.

    I have used quite a few hvac specific controllers - easy to understand, robust, easy to tune, and easy to adapt to....
    Watching how its made and saw all those machines working in sequence with one another and not missing a beat just sparked my curiosity into PLC.

    Trane try to adopt to that mall system and had a pain in the a$$ time doing it.

    The other thing that drew my interest is the fact of picking up a ACD book and ordering whatever you needed versus dealing with ALC or Trane or Novar and having to locate a dealer, pay there percentage for the equipment, and pay for programming if they held the licensing for the system.
    Just trying to find a way around the man!

  7. #7
    Join Date
    May 2002
    Posts
    9,564
    Well, we have a controller that isn't licensed that programs with IEC61131-3. So, if you like ladder logic you can use it. People typically like to use functional block when available.

    Program cycle times down to 10ms.
    No reboot on program changes.
    Run multiple program instances.
    Offline and full online debugging.
    100% backup capability to a SQL DB.
    Redundant power connections.
    Up to 320 physical I/O with each module having backlit LCD display.

    ...anyway, that's some features more "PLC-like". And they can speak BACnet/LON/Modbus/OPC and have alarming, scheduling and trending built in. Scheduling is another feature that usually PLC guys don't care about besides open protocols. So, usually with PLC in building automation they fall short with many features you take for granted in building automation systems.

    Really, all the devices are PLC's. Some just have better features and construction.

  8. #8
    Join Date
    Mar 2003
    Location
    NY
    Posts
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    We recently installed a small project where the customer insisted on PLCs. I had a new kid that was familiar with the programming so we agreed to do it. The costs were staggering to get add-on modules for temperature inputs and analogue outputs. In fact for 2ins or 2outs the cost was almost equivalent to the base module and we could not get combination temp and analogue ins so if we had one temp sensor and one 0-10v in then we needed two modules. Any more than two then we needed an additional module.

    Cost wasn't bad if you are only doing digital ins and outs but it seems like the analogue thing is a whole new world to them and costs get real crazy. I will never consider using a plc again in an HVAC environment unless the customer is first made fully aware of the costs differences involved.
    "Controls is a lifestyle not a job" -klrogers

  9. #9
    Join Date
    Jan 2011
    Location
    Louisville, KY
    Posts
    20
    I have several pieces of equipment that used PLC's as the temperature controller as well as control of the automation. The PID loop programing in a PLC is a real pain. To tune and maintain temperatures with constant changes in the external variables was a constant challange. Had a lot of over shoot , static discharge spikes and ground loop problems that cause temperature faults. Put in some off the self temperature controllers set them and problems disappeared. The PLC runs the machine and the temperature of the pads is nice and steady. The Algorithms of dedicated temperature control equipment tend to be better since each generation has enhancement where as a PLC's advantage is that it is simple to change i/o and sequences and is very adaptable. Some of the newer plcs have higher language such as C++ and others that allow them to work calculations better than the old ladder logic only PLCs so one day the temperature programs may be canned and allow the PLC's to control temperature easier.

  10. #10
    Join Date
    Jan 2003
    Location
    USA
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    How many times have you seen a Cray super computer running a simple calculator?

    Better yet, how many PLCs have full blown scheduling built in?
    Propagating the formula. http://www.noagendashow.com/

  11. #11
    Join Date
    Dec 2004
    Location
    San Francisco Bay Area
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    The only time I've had to program a PLC for anything related to temperature control was a recent datacenter we did, where the chiller plant was considered crucial enough to require redundant CPU's - none of our DDC solutions offered true redundancy, so we went with a PLC brand that offered both redundant power supplies and redundant CPU's.

    I'd never seen such a cool setup - if one of the two CPU base modules takes a dump for some reason, the backup picks up the program where the primary left off - within a millisecond or two, with no hiccup in the outputs. Latching relays offered some additional protection from hiccups, but flicking back and forth between the CPUs without a glitch was neat to see.

  12. #12
    Join Date
    Mar 2005
    Location
    Pacific Time Zone
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    Which PLC brand?
    "How it can be considered "Open" is beyond me. Calling it "voyeur-ed" would be more accurate." pka LeroyMac, SkyIsBlue, fka Freddy-B, Mongo, IndyBlue
    BIG Government = More Dependents
    "Any 'standard' would be great if it didn't get bastardised by corporate self interest." MatrixTransform
    http://threedevilskennel.com/ - not my website.
    Versatile Hunting Dog Federation - www.vhdf.org/


  13. #13
    Join Date
    May 2002
    Posts
    9,564
    Quote Originally Posted by davem View Post
    The only time I've had to program a PLC for anything related to temperature control was a recent datacenter we did,... - within a millisecond or two, with no hiccup in the outputs. Latching relays offered some additional protection from hiccups, but flicking back and forth between the CPUs without a glitch was neat to see.
    We have ability for redundant power but when I see this you could do this with two separated controllers... probably still less money. No doubt, very slick.

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