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DC - DC LVDT voltage comparison

Discussion in 'Electronic Basics' started by, Feb 26, 2004.

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  1. I need to deliver a logic signal based upon the comparison of two dc
    voltages coming from a DC - DC LVDT (sheet ) . These inputs will be in the range
    of +- 5V and are temperature stabilized and filtered. Since the discrete
    movements being measured are ~ .00015", the device will have to be very
    precise. Any suggestions would be appreciated.
  2. Keep in mind that the filtering used to reduce the noise and ripple
    voltage exiting these units will slow their response to movement,
    also. That filtering appears to be specified as high as 10 mv with a
    10 volt output range. So even though the average output resolution is
    infinite, there will be short term errors equivalent to .01/10=0.1% of
    full scale. If your unit has a full scale stroke of 0.4 inch, this
    noise represents a position error of about 0.0004 inch. There are
    also steady state errors caused by temperature change and nonlinearity
    that no amount of filtering can cure.
  3. kdoney

    kdoney Guest

    Thanks John. This system response time can be seconds and I can smooth any
    output with a capacitor on the output. I am using this to check for backlash
    in a ball screw. So I can command a movement of .00015 then wait for a
    noiseless comparison then continue. Wouldn't this eliminate all problems
    except the .25% non linearity within the LVDT specs?
  4. kdoney

    kdoney Guest

    I meant a switched capacitor on the output.

  5. John Fields

    John Fields Guest

    With a full-scale deflection of 0.4" and an uncertainty in the output of
    +/- 0.017% of reading/°F, it seems to me that your error budget's going
    to be shot in just over 2°F. Add to that the zero error of 0.006% of
    full scale/°F and it'll be shot with less than a 1°F change in temp
    unless you've got the thing in an oven or something, so it may be that
    the accuracy/stability of the comparison circuitry is the least of your
    problems at the moment.

    Assuming that I'm wrong and/or that you've got that problem under
    control, when you say "two dc voltages" I'm assuming that you mean you
    want to know the location of the plunger at two points along its travel
    and want to determine that by measuring the voltages which correspond to
    those points. Right?
  6. Right. It will help reduce the random noise and the ripple voltage on
    the outputs but will not make the absolute measurement any more
    accurate. Good for small changes, though.
  7. kdoney

    kdoney Guest

    When I said 2 DC voltages I was referring to the reference voltage (my first
    reading from the LVDT) and the present voltage coming from the LVDT. I will
    move the LVDT with a servo motor controlled ball screw until I get a
    voltage. Then I will reverse the servo, one step at a time (.00015") until I
    get a different voltage. This will give me the backlash. I really only use
    the LVDT to detect movement. I don't mind if the voltages have errors since
    I will be subtracting the reference voltage. The servo movement actually
    gives me the measurement. If the temperature is going to change the reading
    during testing beyond the difference that .00015" movement would make then
    it won't work.
  8. Not sure where you are going to switch the capacitor in. What total
    travel LVDT are you using?
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