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Thread: DDC and PLC

  1. #1
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    DDC and PLC

    Are they the same product with a different name?

    Whats the pros and cons?

    I predominatly have been using DDC controls for the last 12 years.
    I have used a variety of products from TAC, Johnson, Siemens and others.

    I have recently been looking at some PLC product like Siemend LOGO, Crouzet Millenium, Moeller Easy & Titan, Telemecanique Zelio.

    What I have seen has impressed me as far as bang for buck. So much so I might adopt a line and start using them instead.

    What are others thoughts?

    Dazza

  2. #2
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    From the limited research I have done it appears that most DDC vendors (HVAC) have a bunch of pre-built functionality in there systems (schedule, optimum start/stop, enthalpy calculations, command mismatch alarm, pid loops etc.) that do not appear to be in PLC. Not saying it can't be done, just might take a lot of work. PLC appear to be more robust and the scan time might be a little quicker (got to cound those million bottles going down the conveyor belt) and are used in the industrial (petro-chem, pharmaceutical, waste water treatment, bottling plant etc) market more. B&B Electronics have some neat PLC and PLR devices that appear to be extremely cost effective yet basic.

    I would stick to using DDC for HVAC automation and leave the PLC to the industrial boys, I know of one DDC company that took on a PLC waste water project and it was a major contributor to them closing their doors. Not that they weren't a decent DDC company, they just got in way over theri head with PLC and industrial automation.

    kontrol out.
    "Open is as open does." - Forrest Gump
    "Can't we all just get a Lon?" - Garry Jack
    "BACnet: integration or interrogation?" - The Janitor
    "Open protocols? You can't handle open protocols!" - Nathan R. Jessup
    “What’s that? Aaa… open protocols? Don’t talk about…. open protocols? Are you kidding me? Open protocols? I just hope we can hardwire an interface!” - Jim Mora http://www.youtube.com/watch?v=U7fjDS0jKiE

  3. #3
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    All of the products i named have programming by FDB or ladder and have precanned functions like PID etc.

    What I should say is that I am only looking at small scale jobs.

    Dazza

  4. #4
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    That's what I was looking at B&B for, small stand alone project that will never require a OWS. Farm pumps and filtration systems. Most are done with time clocks and pressure switches in the area, having a little PLC would be cool, but the added maintenance would be a little much for the average field worker and the client never went ahead with the proposal.

    kontrol out
    "Open is as open does." - Forrest Gump
    "Can't we all just get a Lon?" - Garry Jack
    "BACnet: integration or interrogation?" - The Janitor
    "Open protocols? You can't handle open protocols!" - Nathan R. Jessup
    “What’s that? Aaa… open protocols? Don’t talk about…. open protocols? Are you kidding me? Open protocols? I just hope we can hardwire an interface!” - Jim Mora http://www.youtube.com/watch?v=U7fjDS0jKiE

  5. #5
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    Whats B&B?

  6. #6
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    They are not usually considered the same although most of the control modules used in DDC do meet the descriptive definition of Programmable Logic Controller.

    A "true" PLC is typically more flexible in what it can control but generally the programming will be more intensive and less intuitive relative to a DDC product. DDC programming typically has "cookie cut" functions already built to support the intended role of the controller.

    The cost of the PLCs can be less but the time required to configure and program can often negate any savings. Of course, once you developed a couple programs (one for typical VAVs, chiller plant, HHW, etc.) they can be copied from controller to controller.

    Very high-end PLCs can be ridiculously expensive.

    PLCs tend to be more "powerful" and as always you can overbuild a control system or network but under building one (e.g. using HVAC DDC to run a power plant) is asking for trouble.

    D1G

  7. #7
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    "Open is as open does." - Forrest Gump
    "Can't we all just get a Lon?" - Garry Jack
    "BACnet: integration or interrogation?" - The Janitor
    "Open protocols? You can't handle open protocols!" - Nathan R. Jessup
    “What’s that? Aaa… open protocols? Don’t talk about…. open protocols? Are you kidding me? Open protocols? I just hope we can hardwire an interface!” - Jim Mora http://www.youtube.com/watch?v=U7fjDS0jKiE

  8. #8
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    PLC's are indeed are used in an industrial type applications. They are used in high speed sorters, conveyor systems, robotics, packaging machines, in instances where timing means everything. They also control servomotors (via fiber optics) that need to stop on a dime, as well as VFD's. Typically the scan rate is very fast on the CPU.

    Also usually there is a CPU device that is attached into a backplane where other cards can be added for different functions (inputs/outputs/combos/network cards). Typically sensors can be RTD's, thermistors, photoeyes, scanners, vision systems, prox sensors, etc.

    Predominately the programming language is ladder logic. However some systems (AB) have GPL (similar to LogicBuilder/Tridium). Other companies use text based (Siemens).

    There are distributed systems also. Networks are used such as DeviceNet, EtherNet, Profibus, ControlNet, Modbus, Lon, and even BACnet (FieldServer makes a ControlLogix card).

    You also have front end systems such as AB which handles AB products, or you can go with WonderWare, that is a Tridium type software application to bring multiple manufactures under one front end.

    In all respects they are almost the same type of system except used in different environments of a building.

  9. #9
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    If I didnt know any better I would say they are the same as the zelio and mellenium controllers, just in a different box.

    Dazza

  10. #10
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    Quote Originally Posted by control_8 View Post
    Also usually there is a CPU device that is attached into a backplane where other cards can be added for different functions (inputs/outputs/combos/network cards). Typically sensors can be RTD's, thermistors, photoeyes, scanners, vision systems, prox sensors, etc.

    Predominately the programming language is ladder logic. However some systems (AB) have GPL (similar to LogicBuilder/Tridium). Other companies use text based (Siemens).
    This used to be my understanding of PLC until I started looking into them more recently. Have a look et the B&B link in the post below and you will get an idea of what I am talking about.

    Dazza

  11. #11
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    Quote Originally Posted by azzad View Post
    This used to be my understanding of PLC until I started looking into them more recently. Have a look et the B&B link in the post below and you will get an idea of what I am talking about.

    Dazza
    It's still pretty much the same thing...instead of plugging into a backplane, it appears they might snap into eachother. In the end, still the same concept (a CPU with expandable I/O or Com)...

  12. #12
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    Quote Originally Posted by control_8 View Post
    It's still pretty much the same thing...instead of plugging into a backplane, it appears they might snap into eachother. In the end, still the same concept (a CPU with expandable I/O or Com)...
    Most importantly not just a cpu but a cpu with IO that is expandable.

    "It's still pretty much the same thing" this pretty much sums up my thoughts on DDC and PLC.

    Dazza

  13. #13
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    One of your biggest issues with selling your customer PLC based systems will probably be training. Most of the PLC languages are not "high level" or intuitive, you don't have the ability of "seeing" component on/off you'll only see output/channel on/off. That is, you often do not have the ability to assign "meaningful" names to I/O, it's not fan status it's "Y1 on/off" or "Y1=1/0". This is normally accounted for at the front end but can be difficult for your average facility person to grasp.

    The other issue to keep in mind is that (as mentioned above) the predominant language is ladder logic. Ladder logic is a great language when you only need binary functions, coding an analog output can be painful depending on the controller and interface. Controllers supporting SFC or FDB may be better suited for you.

    OPC compliant controllers are best IMHO, There are "tons" of very powerful front ends (putting a lot of DDC front ends to shame) you can use with them. Most literally give you the flexibility to do anything you can dream up and code.

    The GPL (not general public license) license from AB is fairly expensive IIRC, maybe worth it but you'll have to decide that on your own. It could be free now for all I know though so take that with a grain of salt.

    If it's not and you don't want to buy it your possibilities are nearly endless especially if you have people with programming (not cut and pasting preformed code or functions) skills or the aptitude to learn.

    D1G
    Last edited by D1G; 11-13-2009 at 01:03 AM. Reason: grammar > me

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