help with plc or temperature controller please

Discussion in 'Embedded Systems and Microcontrollers' started by iswitch, Oct 21, 2012.

  1. iswitch

    Thread Starter New Member

    Oct 21, 2012
    5
    0
    HI

    I am embarking on a project where i need to switch a load (via a contact) depending on ambient room conditions.
    I need to stop condensation build up on a pane of glass in a swimming pool area.
    I need to have remote humidity and air temperature sensors and switch the load when the glass is at lower temperature than the ambient room or the humidity is high enough to cause condensation.
    I have looked at omron products but i need some help on deciding what i need first.

    Can anyone help get me started please.
     
  2. wmodavis

    Well-Known Member

    Oct 23, 2010
    737
    150
    You probably really need the inside temperature to be above the dew point don't you?
     
  3. iswitch

    Thread Starter New Member

    Oct 21, 2012
    5
    0
    Ideally yes.
    I would need to make that calculation on the fly from the readings of temperature and humidity sensors.
    So I assume I would need a small plc to do this calculation and switch the output, I would then need to check the readings and make the calculations say every few minutes to determine what state the switch should be.
    Can you recommend a plc device to achieve this?

    Thanks
     
  4. wmodavis

    Well-Known Member

    Oct 23, 2010
    737
    150
    Pick one that will accept the sensors you will use and be able to perform the calculations you need and control the load you will be switching. There is likely a ton of them out there. Cost certainly enters in and familarity with the programming and interfacing, if you have them, might influence your selection.

    As the 'project engineer' you need to list your requirements then review the product capabilities and tradeoffs then decide. You have provided only a miniscule subset of the information I would use to make that decision so recommending a make/model would be pure guessing.
     
  5. MMcLaren

    Well-Known Member

    Feb 14, 2010
    759
    116
    I'm curious if anyone has come across any projects that perform a similar function using a relatively inexpensive microcontroller board along with a temperature and humidity sensor, like the DHT11?
     
  6. iswitch

    Thread Starter New Member

    Oct 21, 2012
    5
    0
    wmodavis apologies for being so vague.
    I have not dealt with controls before so I am trying to gather as much helpful info as possible to help me decide what I need.
    If I knew the information you suggest I probably would not have come here in the first place, but thanks for taking the time to read and reply anyway.

    It would appear that dew point transmitters are available. This may save me some work.
    So I need to find the connections for a dew point transmitter that meets my needs and then find a plc or controller that will work with said connections.
    I also need to make sure the controller will work with the correct voltages for inputs and outputs.
    Ideally the controller would have a script like interface to program the control logic.
    The controller needs to be din rail mounted inside a panel.

    If anybody has any experience in what I am trying to do here and is prepared to share some advice it would be much appreciated.

    Thanks
     
  7. iswitch

    Thread Starter New Member

    Oct 21, 2012
    5
    0
  8. panic mode

    Senior Member

    Oct 10, 2011
    1,318
    304
    You just posted links to main pages of two companies with hundreds of products. You need to choose sensors based on your application needs. What ever you choose will have output of some sort.

    If you are interested to connect those sensors to a PLC, you better choose sensors that have outputs compatible with PLC inputs. Normally those would be analog inputs such as 0-5V, 0-10V or 4-20mA.
    Due proximity to water, you probably also want something that is sealed and rated IP67 or at least IP65.

    You may find combo sensor like
    http://www.michell.com/uk/products/pcmini52.htm
    which would simplify installation and wiring.

    Few hundred pounds is not much for industrial grade products. It may buy you sensor(s) and cables, maybe a controller too if you go for low end ("smart relay" instead of PLC). But choose wisely, you will need controller with appropriate inputs. If your controller does not have them, you will also need to invest into analog input card which will not be cheap.

    Finally everything will need some sort of power supply. Any time you get something integrated you save cost (cheaper than buying separate items). Some controllers have built in AC power supply and expose 24VDC output terminals. This is enough to power sensors and maybe a relay or two (if the controller outputs are not relays).

    You may want easy access to the setpoints to make possible adjustments on the fly, without having to use laptop with programming software. Some controllers have minimalistic interface built in (cost saving) but again watch for IP ratings. Separate HMI costs more.

    Another thing to consider is software. If you don't have it already it is likely going to cost you. PLC programming software is not free. Programming software for low end products (smart relay) may be free (Zelio, Click PLC, etc.). CoDeSyS is free and can be used to program variety of 'real' PLCs but libraries for particular brands may not be free.

    You should do some research first as you are likely to find products better suited for the job (high level of integration). If you are going Omron route, you are probably thinking CP1 or ZEN family (such as ZEN-10C1DR-D-V2). I would probably also consider custom design (MCU) or something like Panasonic FPe, Moeller XVC100 or similar. If customer likes to monitor things using phone etc. maybe something Nano-10 (http://www.tri-plc.com/nano10.htm)
     
    Last edited: Oct 28, 2012
  9. iswitch

    Thread Starter New Member

    Oct 21, 2012
    5
    0
    Hi panic mode

    Thank you very much for all the helpful information in your post, much appreciated.
    I will update this post with my results for those that may be interested.
     
  10. John P

    AAC Fanatic!

    Oct 14, 2008
    1,632
    224
    If this were a student assignment or something you're doing essentially as a hobby project, I'd suggest making the window into a dewpoint sensor. You could have an infrared LED flashing at some frequency, placed so that the light reflects off the window at a low angle. Then there'd be a receiver which picks up the reflected light, and a controller which evaluates whether the intensity is being reduced by condensation on the glass, and which turns on the heater if necessary. That actually is the way dewpoint sensors (some of them, at least) work, and of course if you built it this way, you'd be getting feedback from the actual item you're interested in, namely the window, rather than some external sensor.
     
Loading...