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4-20mA scaling ???????

Discussion in 'Electronic Design' started by Fred Bloggs, Aug 17, 2004.

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  1. Fred Bloggs

    Fred Bloggs Guest

    You can make an active current divider with R2 a variable resistor like
    so, it can be linear enough for small scale changes:

    View in a fixed-width font such as Courier.

    Iin x -------
    R1 + R2
    Iin ---> vfd input
    ---> +-----------+
    R1 | |
    | | | | |
    4-20mA | | | | |
    loop | | | | |
    | | | | |
    <------|-------------------|----+---+ |
    | | | |
    | | +-----------+
    | |
    | |
    | / | |
    | R2 / +|--+
    +----/\/\--+-< |
    | \ -|--+
    | \ | |
    | |
    | |
  2. Tim Wescott

    Tim Wescott Guest

    (a) Use a panel-mount PLC. Overkill, but you'll have lots of useful

    (b) Post your question on sci.engr.control. There's folks there who
    know _way_ more about industrial control than I do, and will be able to
    help you out with finding a simple, rugged & maintainable adjustment
    device to work on current loops.
  3. Pablo

    Pablo Guest

    I am working on a project where I have 1 DC drive which outputs a 4-20mA
    signal to a VFD, I want the VFD to follow the DC drive which it does,
    however I want to be able to scale the VFD. That is if the DC drive is at
    say 50% then the VFD would be at 50%, but, in some cases I want the VFD to
    be say 45%. I have gone through the programming and found that the DC drive
    has no means of scaling the 4-20mA signal and the VFD would be too
    complicated for my production people to program each time they wanted to
    change scaling. Is it possible to adjust the 4-20mA signal in some way, say
    with a potentiometer or some device. I would like to do it this way so as to
    make it simple for my operators. This setup is basically a machine which is
    driven by a DC drive and a colour feeder which sends a pigment into the
    machine, when we push the output up or down on the machine the colour must
    follow, but as you may have guessed the colours are fed at different
    percentages. Thanks to all whom reply. Paul
  4. nospam

    nospam Guest

    Trouble is that will also divide the 4mA offset. Dividing by 2 would give
    you 2..10mA not the 4..12mA you want.

    If the VFD has a voltage input then a commercial 4..20mA -> 0..10v
    converter and a pot is probably as simple as it gets?

    Also even cheap VFDs are getting quite sophisticated, I'm sure some will
    have the facility to scale one analog input with another.
  5. Ban

    Ban Guest

    Pablo, the best would be to convert the signal back to a voltage, scale this
    with a pot and create again the 4-20mA current. There are ICs dedicated to
    this task. AD694 comes to mind. This is a very precise and reliable method
    with well defined output current, that works with every transducer and
    connected data logger under all circumstances.
    You could of course add or subtract a certain current with a bipolar current
    source as well, which would require more components.
  6. Jim Thompson

    Jim Thompson Guest

    Fred, You also moved the zero point.

    ...Jim Thompson
  7. There are analog building blocks that can be put together to do this
    sort of thing. They are industrial parts that accept and output 4-20
    ma signals.

    Take a look at the catalog from an example such as Action Instruments.
  8. So much depends on how the existing input behaves, and the output is used.
    At the 'simplest', the input may well have a fixed DC resistor, across
    which the voltage is measured to sense the current. In this case, the
    current can be effectively scaled, by simply paralleling a second resistor
    with the existing one.
    However there is then a significant 'caveat', in that this moves the
    entire 'range', so (for instance), if the input had a 100ohm resistor, and
    a 1400ohm resistor was paralleled with this, the measurement at 12mA,
    would reduce by 5%, but the new 'zero' point, would become 4.28mA, rather
    than 4mA. Also some sensors use an overcurrent (normally at about 22mA),
    as an 'alarm' indication, with the current reduced like this, the
    overcurrent value would no longer be properly sensed.
    To do the job 'properly', requires a lot more care, with probably a
    circuit to subtract the 4mA offset, scale the value, and duplicate any
    'special' values like the overrange signal, then add back the offset...

    Best Wishes
  9. Pablo

    Pablo Guest

    Most of the time we will be mid scale(like 12 mA) , thus if the bottom end
    is affected this does not matter, I just want to add a pot or something so
    that we can add the pot setting to a setup manual for a particular product.
    Thus the operator can do a setup and simply dial in the pot setting from the
    manual and know that it is the same as the last time and the percentage of
    color being added is the same. I was thinking of a three digit pushbutton
    type pot whereas the setting is very easily recorded and set.
  10. Pablo

    Pablo Guest


    Not sure I understand the device beside R2, can you exagerate. I am thinking
    that this would work but I am a little confused. Also what value would one
    use for R1. Thanks Paul
  11. It's pretty easy to do this sort of thing electronically, but I
    suggest you just buy a module that will be protected against reversed
    inputs, have a power supply (if necessary), have a reasonable
    frequency response (and rolloff) and all that practical crap that
    makes it into work.

    Best regards,
    Spehro Pefhany
  12. Rich Grise

    Rich Grise Guest

    Google "current divider".

    Good Luck!
  13. Fred Bloggs

    Fred Bloggs Guest

    This impractical thing will take care of that offset:
    View in a fixed-width font such as Courier.

    .. CS R2
    .. +-----+ (Iin-4mA)x------- + 4mA
    .. +--|4mA->|--+ R1 + R2
    .. Iin | +-----+ | vfd input
    .. ---> | | +-----------+
    .. | R1 | ---> | |
    .. >------+---+---/\/\----+---+--------------------+---+ |
    .. | | | | |
    ..4-20mA | | | | |
    ..loop | | | | |
    .. | | | | |
    .. <------|-------------------|--------------------+---+ |
    .. | | | |
    .. | | +-----------+
    .. | |
    .. | |
    .. | / | |
    .. | R2 / +|--+
    .. +----/\/\--+-< |
    .. | \ -|--+
    .. | \ | |
    .. | |
    .. | |
    .. +--------+
  14. It sounds like you really need an operator interface panel that
    provides this minor fiddling of the process signals- something that's
    repeatable and easy/error-free to set up for each job. In retrospect,
    the modules that I and others have suggested might not be the best for
    that purpose.

    Are you doing compounding of polymer resins? Is the environment
    extremely dusty?

    I might be able to help with this little project, feel free to contact
    me if you'd like.

    Best regards,
    Spehro Pefhany
  15. It's pretty easy to do this sort of thing electronically, but I
    I make such modules for a customer. They include a PIC18F252, have two
    current inputs and one output, and can perform any computation
    inbetween. Also a character LCD. But these are not cheap :)

    Wouter van Ooijen

    -- ------------------------------------
    PICmicro chips, programmers, consulting
  16. Rich Grise

    Rich Grise Guest

    Spehro Pefhany wrote:

    Geez! Is _everybody_ out of work?

  17. Roger Gt

    Roger Gt Guest

    : Spehro Pefhany wrote:
    : ...
    : > I might be able to help with this little project, feel free to
    : > me if you'd like.
    : ...
    : Geez! Is _everybody_ out of work?
    : :)

    Pretty much.... I know 37 unemployed Engineers myself!
  18. Rich Grise

    Rich Grise Guest

    I guess it's a good thing they needed that new drywall taped today then. ;-)

  19. Pablo

    Pablo Guest

    I have now two options (that is bought type options) and I am waiting for
    salesman on the one. First is a KB product which does exactly what I need it
    to, however at a cost of 175.00 canadian, add that with a good ten turn or
    pushbutton pot which cost about $50-75, thus overall cost would be about
    250.00 per machine. We are looking at 8 machines. The other option which is
    getting me firm is to use a operator interface which Control Techniques
    sells and use it to scale the current output.
  20. What model number is that?

    Best regards,
    Spehro Pefhany
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