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Reading Hundreds of analog voltages

Discussion in 'Electronic Basics' started by convergence, Jun 30, 2005.

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  1. convergence

    convergence Guest

    Hi,
    I have to measure the voltages of hundreds of batteries and display
    them on a PC. For the start the number is 200. 20 batteries in series
    form a group and we have 10 groups in total.
    I need 400 differential Inputs. The old technology we are replacing
    uses Relays. If the user selects the Relay Group, the analog voltages
    of this group can be read out.
    I want to replace the Relays with ICs. Any suggestions? Should I use
    multiplexers?
    Thanks
     
  2. Bob Masta

    Bob Masta Guest

    You will undoubtedly want to look at multiplexers.
    But one thing to be careful about is the common mode range
    of your differential inputs. You don't say what voltage each
    of the 20 batteries in series is, but even if they are single cells
    you are talking about more than 30 volts. Make sure your
    multiplexers can handle the entire series voltage. If each
    battery is more than one cell, you may want to think twice
    before you get rid of the relays.


    Best regards,


    Bob Masta
    dqatechATdaqartaDOTcom

    D A Q A R T A
    Data AcQuisition And Real-Time Analysis
    www.daqarta.com
    Home of DaqGen, the FREEWARE signal generator
     
  3. John Larkin

    John Larkin Guest


    Use leapfrogged solid-state relays; odd nodes switched to one meter
    node, even ones switched to the other.

    John
     
  4. Chris

    Chris Guest

    ----------------------------------------------

    Because of the high voltages which are probably involved, it might be
    better to stick with the old technology. Reliability of most relay
    multiplexers of this type can be improved with simple inexpensive
    additions to the ATE, and setting up a preventative maintenance
    schedule to replace the relays on a regular basis.

    ----------------------------------------------

    Hi. I'm assuming you're not satisfied with the reliability of your
    relay multiplexer, and you're looking at replacing it with a solid
    state system (based on analog switches) in order to improve
    reliability.

    Before you walk away from the relay concept, you should be aware of its
    advantages. Open is really open (100s of megohms or gigohms between
    contacts). If you've got a good relay contact, closed is really closed
    (milliohms). You're not going to get that ratio between open and
    closed with any analog multiplexer IC. Not only that, but an analog
    switch also has leakage currents between switches on the same IC, which
    isn't a problem at all with relays. And as long as the relay contacts
    are rated to withstand voltages greater than that seen in your battery
    system, you don't have to worry about finding high voltage multiplexer
    ICs. You didn't mention the voltages you're measuring, but that could
    be a serious problem.

    Another big advantage is that relay contacts are immune to ESD
    (electrostatic discharge). People who switch to solid state
    multiplexers have to deal with the fact that they're now working in an
    anti-static environment. Static can easily kill many analog
    multiplexers. Of course, many analog switches are made with ESD
    protection, but you always pay for that with a fairly dramatic increase
    in leakage current and usually with a decrease in the ratio between off
    resistance and on resistance.

    If reliability is your only issue, you might be able to improve that
    quite easily and keep a working system running without the hassle of
    redesign.

    First, you should look at the type of relay you're using. Since you're
    switching into a multimeter, you should choose a relay that's made for
    "dry switching", meaning that there's not enough current or voltage to
    wet the contacts on make or break (the "cut" on the data sheet is
    usually 5 V at less than either 1 mA or 5mA). Standard relays won't
    work well for test. You didn't mention the type of relay you're using,
    so that's one thing you might want to look at. If the minimum
    switching current isn't specified on the data sheet, you can call the
    relay manufacturer to find out. Reed relays are generally OK for dry
    switching, but you should check. If you don't have reed relays, ones
    with bifurcated contacts are frequently capable of doing this job.

    If you have reed relays, you should also check the withstand voltage
    specified on the data sheet. Many reed relays are only specified to
    switch 100VDC or less. Again, something to check, since you didn't
    mention which relays you're using or the voltage you're switching.

    Another issue which might be a problem is the battery environment,
    which sometimes has a very acidic and corrosive atmosphere. This
    affects relay contacts as well as any other metal. If this is an
    issue, it also affects electrical connections and such, and should be
    addressed first. This will be a killer no matter what you do. Sealed
    relays might give you better results, but it's better to improve the
    environment. Another option would be to place the test equipment in a
    remote location with good ventilation. These are basically DC
    measurements, so a couple of hundred feet of cable shouldn't affect
    your measurements too much unless you're measuring individual voltages
    in a millisecond time frame.

    Every reed relay manufacturer specifies an open contact capacitance, as
    well as a capacitance between contact and coil. Usually this is only
    several pF, which isn't much of a problem if you're just using just
    one. But if you're using hundreds of relays in parallel, 4pF can be
    multiplied to the point that it's a big issue. You haven't described
    the layout of your test setup, but your description implies that you've
    got 10 banks of 40 relay contacts, each of which go to another set of
    20 DPST contacts. You might want to look at the schematic of your test
    layout, and imagine a several pF cap in place of each N.O. contact.
    You'll see that, in fact, you're switching hundreds of pF of
    capacitance when you're switching voltages. That changes things quite
    a bit, because now you're talking about surge currents that are limited
    only be the resistance of the wire and the relay contacts. That can
    easily reduce the life of the relay contacts and make a very unreliable
    system. This is valid no matter what the input impedance of the meter,
    which is generally a DC current and a capacitance added to the above.

    Looking again at the data sheet of the relay, you'll see a maximum
    specified current, along with a specified number of electrical
    operations at that current. Generally, you'll also see a specified
    number of mechanical operations (at essentially no electrical current).
    The first number for reed relays can be as low as 50,000 to 100,000
    operations, and the second is usually in the millions. At a bare-bones
    minimum, you should add enough series resistance to the test line so
    the peak switching current for the capacitive load is less than the
    specified maximum relay current. Usually that means adding a small
    series resistor between each voltage and its relay contact. The less
    current you switch, the better. I like to start with a series resistor
    that limits current to 1/10th of specified maximum or less, if I can.
    I'll then somewhat arbitarily call the expected life at 1/2 to 1/5 of
    mechanical maximum (depending on balance between maintenance costs and
    cost of bad readings).

    You have to take a good look at the load impedance of the meter here.
    If you've got a very high (e.g. 10 Megohm) input impedance meter,
    adding a 2K series resistor (0.5 amp max. reed relay contact current,
    set resistor for 50mA pk. at 100V) shouldn't cause you any problem.
    Your voltage readings will only change by 0.2%, which isn't a problem
    for most ATE applications. If it is, you can compensate with software.
    But if you've got a load resistor at the meter (which may affect the
    above calculations of switching current a bit), or have a low input
    impedance meter (many DVMs have different input resistances for
    different ranges), you may have a voltage divider between the series
    resistor and the meter. Your best bet here is to obtain 0.1% resistors
    (they're cheaper by the hundred) to minimize the resistance
    uncertainty, and compensate in software for the voltage divider.

    Now it's time for a little math. Take a realistic look at how many
    cycles you're putting the relays through per unit of time, and
    extrapolate that to find out how frequently you have to replace your
    relays as preventative maintenance. Your inside set of 20 relays will
    cycle 20 times for every time the outside 400 cycle, so you'll have to
    replace the inside set of relays 20 times as frequently. This number
    will give you a good idea of the maintenance costs of your relay
    multiplexer system per test cycle and per year.

    I found it useful in systems like this to get a small Omron counter
    with LCD display, voltage input and internal 5-year battery, and place
    the increment input of the counter across one of the outer ring coils
    (they're on longer, so there's no problem with activation time). I
    would then document that the counter be periodically checked, and the
    relays be replaced on a regular schedule.

    If the existing relay system is older or has been well used, the
    contact resistance of the relay sockets may be a bit of a problem.
    This is especially true if people have been swapping out relays at
    random in frustration. By putting together a test program to measure
    resistance of closed contacts with new relays, you can get a good
    handle on the status of the relay sockets with several
    insertion/extraction cycles (reseat relay, measure short circuit
    resistance, repeat). If some show up bad, it might be best to replace
    all of them. The good thing is that, if you have a rational
    preventative maintenance cycle, you limit the number of socket
    insertion/extraction cycles to a minimum.

    By this point, you've got a good handle on how to make your automated
    test setup very reliable, as well as getting a handle on how much
    maintenance of the wear parts (relays) will cost to keep it reliable.
    Having that, you can make a judgment on whether to go with analog
    multiplexer ICs and the failure mechanisms associated with them.

    If you want more follow-up information, please include the following:

    * Maximum voltages switched

    * Floating or grounded batteries or meter

    * Type of meter used

    * Load resistance at meter and load resistor (if any)

    * Type of relay used

    * Measurement frequency (once per millisecond, second, minute, hour?)

    Moral of the story -- old ain't necessarily bad. ;-)

    Good luck
    Chris
     
  5. Wood Butcher

    Wood Butcher Guest

    Check out the INA117. It was designed with this
    application in mind. Note fig 14.

    http://focus.ti.com/lit/ds/symlink/ina117.pdf

    It has a gain of 1 and an input common
    mode range of +-200V. Inputs are protected and can
    withstand +-500V. The downside is that the input
    impedance is 400K ohms and will load the cells slightly.

    Art
     
  6. [...Snip masses of useful info...]

    If you are using conventional relays, it will also improve their life if
    they are physically mounted so that the gap between the contacts is
    vertical. This allows any dirt or particles worn off the contacts to
    drop away after a few operating cycles.

    In electromechanical telephone exchanges, this was always good design
    practice.
     
  7. mike

    mike Guest

    I haven't thought this all the way thru...you probably need different
    parts to get accuracy, but here's the concept.
    Configure a LM3900 op-amp as an integrator.
    Use HV fets to connect even numbered batteries to the positive input.
    Use HV fets to connect odd numbered batteries to the negative input.
    Use series resistors to scale the currents appropriately.
    two threshold detectors on the ouput. Use shift registers or a
    processor to leapfrog the
    fet switches up the chain generating a triangle (sort-of) as you go.
    Measure the triangle half periods to get the voltages.

    I'm sure it's more complicated than that, but it's a place
    to start thinking.
    mike

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  8. Guest

    Hi, common mode voltages are allways going to be a headache. An
    alternative is to use a small micro to measure a small group of cells
    powered by the cells it's measuring. Each processor is connected to the
    next with an opto coupler in a long daisy chain forming a serial link
    which connects to the PC. The micro just measures the cells its
    connected to coverts it into an ascii value adds a cell number and
    sends it down the link.
     
  9. convergence

    convergence Guest

    Hi, the batteries can be connected in series which makes 2.0V * 200=
    400V max. The max. differential voltage will be 2.0V. I don't know if
    the INA117 solution is possible for these voltages . If I use the
    relays, The voltages of 10 series batteries will be switched to a micro
    for conversion. An Optocoupler has to be used to send the data to the
    PC.
     
  10. convergence wrote...
    If this is a one-off job and cost is an object, your best bet is to
    obtain a commercial DAQ + relay scanner on eBay. You should be able
    to get what you need for under $500, and start measurements pronto.
     

  11. Or use a resistive voltage divider for each input that exceeds the
    safe input voltage of the Mux and multiplying the reading in the
    software.
     

  12. Didn't National Semi quit making the LM3900 a few years ago?
     
  13. Wood Butcher

    Wood Butcher Guest

    Since you can connect any cell to your A/D with relays I assume
    the battery stack is floating. If this is true you can reference the
    center of the battery stack to ground. That would give you +-200V
    to the inputs of the INA which is within the spec range.

    Art
     
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