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Piggyback meter readings electronically

  1. Oct 12, 2011 #1

    I am wondering if it is possible to "piggyback" off existing meters/indicators in a diesel generator?

    The generator has traditional analog 'VU' meters for voltage, current, and frequency. It also has standard analog guage fuel meter, oil pressure, water temperature, rpm, and lamp/led indicators for overspeed, low oil pressure, low fuel level, battery charge, high water temperature and earth leakage.

    Ideally I would like to monitor all these readings, I have the MOXA AD convertors with analog (0-10V or 4-20mA) and digital (Logic 0 = 0-3V and Logic 1 = 10 to 30V) inputs.

    If I 'piggyback' off these device taking for example the voltage range that is applied to the fuel meter or the voltage applied to the lamp indicators will it affect the circuit? I realise that for the ammeter I would have to put it in series, if the current range is higher than the MOXA input I can just use a current transformer right?

    I have the wiring diagram and the single line diagram but not the full schematic for this Japanese made generator. One colleague I spoke to said to not mess with the existing circuit and just add in the required metering (i.e. a new modbus output fuel meter etc), but it seems a waste when the relative signals are already there?

    Is this possible?

    This is what the panel look like:

    This is the back of the panel:
    Last edited: Oct 12, 2011
  2. jcsd
  3. Oct 12, 2011 #2


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    It is certainly possible to monitor these functions albeit you will probably need to add transducers and/or do some signal conditioning in order to make use of your Analog voltage and current loop inputs. And you will probably need a power supply and/or relays in order to make use of your Digital inputs.

    National Instruments has a http://zone.ni.com/devzone/cda/tut/p/id/3536" that should be helpful.
    Last edited by a moderator: Apr 26, 2017
  4. Oct 12, 2011 #3

    jim hardy

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    dont go hooking up wires until you understand exactly how the generator instruments work.

    same for that Moxa gizmo. i looked attheir site but didn't find anything on their I/O gear.

    two things to watch out for:

    1. A/D converters usually use a multiplexer to select the signal to be digitized.
    it would be inordinately expensive to have one A/D converter (ADC) per signal so usually one ADC serves at least sixteen signals.
    inexpensive ADCs and multiplexers use FET switches which require that input signals be electrically near earth ground, typically ten or fifteen volts.
    If the signals are not near earth ground the multiplexer is unable to block them and current can get from one instrument loop to another through the multiplexer. That fouls up your indication.
    read your Moxa manuals with an eye out for "common mode voltage" and make sure it's compatible with your panel.
    Understand how (indeed whether) your panel signals are earthed.

    2. inexpensive multiplexers go electrically "soft" when the power to them is turned off.
    By "soft" i mean their input impedance becomes low because without power the FET's lose ability to block current, so they load down the circuits they are connected to which for you is your panel indication.
    It is very embarassing to have your indication go haywire because a computer quit. Especially if you're the guy whose name is associated with the computer.
    so read that Moxa literature with an eye out for " power off input imedance".
    One can spend a little more money and buy multiplexer IC's that keep their impedance with no power but i know of only one manufacturer with the foresight to use them. Probably there's others though.
    that shouldn't be a problem with a relay multiplexer but those are expensive and comparatively rare.

    just saying look before you leap.

    old jim
  5. Oct 12, 2011 #4
    I'm actually using the Moxa ioLogik E1200 Series - Daisy-chain Ethernet I/O for Distributed Data Acquisition Systems (http://www.moxa.com/Event/Net/2009/ioLogik_E1200/" [Broken]) series particularly the E1242 and E1210, the product specifications for the analogue inputs are 10M input impedeance, and the digital inputs are NPN, PNP, and Dry contact with 3K VDC or 2K Vrms isolation. The output of these devices is Modbus TCP/IP. Surely the 'lamp' indicators can be monitored as these are just on/off? As I said particularly for the analogue voltage input with 10M input impedance isn't it just like putting a multimeter across the terminals, and what about using a voltage divider if the range is greater than 0-10V? Obviously Ill have to calibrate!
    Last edited by a moderator: May 5, 2017
  6. Oct 12, 2011 #5

    jim hardy

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    sounds like a robust front end .

    their 2kv isolation is a good indicator they have got their feet wet before.

    if analogue impedance is 10 meg with power off it's like a dmm, yes,
    and probably you're okay
    you can check that easily enough with an ohm-meter, just measure an input with your meter on ohms

    from their website it looks like genuine industrial grade gear and probably okay.
    but still i'd call them and ask about comon mode voltage for analog inputs, for it wasn't mentioned on that 'specifications' page..
    with their low analog sample rate it may very well be a relay multiplexer.

    thanks for the reply and i'm glad you looked.
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