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Non-linear functions ...

Discussion in 'Electronic Design' started by Richard Rasker, Nov 24, 2009.

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  1. Hi all,

    I'm currently working with a mass air flow sensor (a Honeywell AWM3100V, see
    http://datasheet.octopart.com/AWM3100V-Honeywell-datasheet-57019.pdf), and
    I would like to convert the rather non-linear response curve of this device
    into a voltage which bears a linear relationship to the actual air flow.
    Ideally, I would like to see the air flow converted in millivolts, so that
    it can be fed into a 3.5 digit voltmeter directly.

    These are the values (F=flow):
    F (ccm) Vout (V)
    0 1.00
    25 1.90
    50 2.67
    75 3.27
    100 3.75
    125 4.17
    150 4.50
    175 4.80
    200 5.00

    The first problem was simple: finding a suitable mathematical function which
    fits the curve; I looked at something along the lines of
    Vout=c1*(1-e^(-F/c2))+1, and it turns out that c1=5 and c2=125 provides a
    near-perfect fit. The second problem was to find an inverse function -- no
    problem there either: F=-c2*ln(1-(Vout-1)/c1) -- leading to the third and
    rather trickier problem, which of course is to implement that inverse
    function in an actual circuit.

    I've been doing some trial-and-error experimenting with a simple circuit,
    based on a simple Si-diode with some bypass and series resistors in several
    configurations, but that doesn't produce satisfactory results -- the best
    curve I get is easily 10% off at the extremes, and that's even without
    temperature instability. All this is of course no surprise, as the
    exponential function of a forward-biased diode is something different than
    a logarithmic function, and a simple PN junction has a temperature
    coefficient of approximately 2 mV per degree Celsius.

    Does anyone know of designs which provide a better fit for this type of
    logarithmic function, and preferably a better temperature stability?

    Thanks in advance, best regards,

    Richard Rasker
     
  2. Joerg

    Joerg Guest


    I'll second that. Being an analog guy this was hard to write but we've
    got to know when to fold'em.

    The MSP430F2003 has the ADC part built in. Somehow DVM tells me it's
    slow so you could get by with the el-cheapo MSP430F2001 or an even
    cheaper PIC. The MSP can work sans crystal. Program it to do dual slope
    conversion for the ADC and PWM the output, then RC filter that. The math
    goes in between where the code is :)

    Piecewise linear can be ok but with diodes it's temperature-sensitive
    and with comparators it's expensive.
     
  3. Jon Kirwan

    Jon Kirwan Guest

    You were looking for an inverse function of something with an offset
    built into it. Sometimes, it helps to first subtract the offset (the
    1V) before feeding it to an inverse function to linearize. That was
    my first thought. Second, was to wonder if you might instead focus on
    a function block that replicates the exponential behavior of the
    sensor but is _driven_ by a linear parameter (such as time or
    frequency, for example) and then arrange things to adjust the linear
    control so that the two outputs match and then read off the control
    parameter value, instead. Third was to consider recommending a micro,
    which is a rather common approach to conditioning sensors these days.

    Jon
     
  4. I assumed that this step could be considered trivial -- in my measuring
    setup, I have a +1.00V reference voltage for this exact purpose.
    Well, of course that would provide a limited (in time and range) solution,
    but I'm rather more interested in a direct transfer function without
    translating the nonlinearity into another domain first.
    I know. But I'm one of those old school die-hards who prefers hooking up a
    dozen or so components to a meter in an hour or so instead of spending a
    multiple of that time programming a controller to do the same.

    Richard Rasker
     
  5. Good questions. I varied the temperature of the air input to the sensor
    between 15 and 60 degrees Celsius, and found no significant differences.
    I didn't vary the air pressure, but the setup is intended for use in normal
    atmospheric pressure at sea level, so this latter parameter is not
    important.
    Also, the gas measured is air with only very low traces (< 100ppm) of
    contaminants, so I don't have to take differences in gas parameters into
    account.

    Richard Rasker
     
  6. Definitely use a micro-- you can implement your equation (in a very
    straightforward manner if you use C) and/or do a polynomial fit to
    squeeze out the last bit of error.

    For example:

    Flow (ccm) = 0.6629*v^5 - 9.5389*v^4 + 53.6729*v^3 - 139.4424*v^2 +
    192.9420 *v -98.3132

    .... which can be evaluated with only five multiplies and no
    transcendental operations.

    You *could* go looking up how to design analog log/antilog amplifiers
    with matched transistors and thermistors for temperature compensation,
    but AFICR this is not 1980 and a suitable micro is going to be
    cheaper, simpler and much more stable.
     
  7. Jon Kirwan

    Jon Kirwan Guest

    It is trivial. But I didn't see you mention it and it's hard to know
    what to assume, from my end. I just documented by early thoughts upon
    reading.
    I'm worried about what appears to be an open-loop approach you are
    taking.
    Well, I'm not going to argue with you about that. You know your
    situation and desires better than I do. However, ... you may find
    yourself buffeted by practical forces to head this way, all the same.

    Jon
     
  8. Joerg

    Joerg Guest

    With dual-slope conversion at the input and a PWM at the output that
    should be quite easy. The 3-1/2 digite DVM probably doesn't read more
    than a few times per second anyhow.
     
  9. Frank Buss

    Frank Buss Guest

    Combine this one:

    http://www.frank-buss.de/PICPWM/

    with some of the C code from this one:

    http://www.frank-buss.de/vco/

    and you're done, in less than an hour :)

    The function is already known, or you can use a simple piecewise linear
    approximation with your measured values. Another idea would be to use a
    slightly more powerful PIC and a realtime spline interpolation:

    http://www.frank-buss.de/spline.html

    I guess it would fit in an PIC16F628 and even with small PICs this would be
    much faster than you can see the changes on your voltmeter. But I would use
    a 7 segment display anyway, driven by the microcontroller, or with a
    MAX7221, which is really nice for this, if price doesn't matter.
     
  10. Rich Grise

    Rich Grise Guest

    .-----. .-----. .-----.
    | | | | | |
    problem o---->| ADC |----->| LUT |----->| DAC |-----> answer
    | | | | | |
    '-----' '-----' '-----'

    ;-)
    Rich
     
  11. Frank Buss

    Frank Buss Guest

    But the requirements were lower limit and upper limit for the voltage,
    which would require more parts.

    BTW: The new ATtiny10, mentioned in comp.arch.embedded, is interesting for
    such applications, because it has some interesting PWM modes and other
    timer goodies, which makes it even more trivial to solve such problems.
     
  12. I don't know but if I had to do it with analog I might simply do averaging.
    This will produce a spline approximation:

    for example:

    75 3.27
    100 3.75

    if your input was 80 then your output is (3.75 - 3.27)/(100-75)*(80 - 75) +
    3.27. i.e., fitting a line between the two points.

    now, you implement the analog electronics to compute that equation then use
    a multiplexer to select the different data points which is simply setup by
    using appropriate resistors that feed your analog equation calculator.

    For a large number of data points this is out of the equation as also if
    linear approximation is bad. (you can use better approximations but the
    analog circuitry starts to get very complex real quick)

    You gotta decide when the analog method starts to become more complex than a
    uC. Alsoa uC will, in general, be much more accurate and be easily changed
    even after fab.
     
  13. Of course the microcontroller-based approach proposed in previous posts is
    very probably the easiest solution. On the "nearly analog" way there are
    some chips dedicated to signal conditionning and sensor linearization, like
    the PGA309 from TI (see http://focus.ti.com/lit/ds/symlink/pga309.pdf).
    However it will probably not be easy to use it in your application as the
    liearisation is done through a modification of the sensor excitation and is
    adapted to wheatstone bridges. But may be you could add an external aop or
    two ?

    Friendly yours,
    Robert
    www.alciom.com
     
  14. A variant may be more easily adaptable to your application :
    http://focus.ti.com/lit/ds/symlink/xtr108.pdf
    Robert
     
  15. First Google hit is this PDF, which has all the equations needed.

    http://www.electronics.dit.ie/staff/ypanarin/Lecture Notes/DT021-4/6LogAntiLogAmplifiers.pdf

    see around page 9 (simplified schematic). Matched transistors are used
    to compensate for offset change with temperature, and to compensate
    for gain error with temperature R4 is made to be a temperature
    dependent resistor. Probably good enough for your application, but not
    as good as the dedicated chips were.

    Unfortunately, the IC dedicated analog blocks they talk of, and even
    many of the matched transistors, have been becoming more and more
    availability-challenged. The market has spoken.
    That does seem a tad on the warm side, Bill. At 22-23°C+ some folks
    start to complain, IME.


    Best regards,
    Spehro Pefhany
     
  16. Ah, yes, of course -- I searched for "non-linear amplifiers", and got lost
    in all sorts of interesting but (for my application) useless information.
    And I still have a few SSM2210's around somewhere, so a matched transistor
    pair is no problem.
    I know, and I guess I'll eventually switch to using controllers for these
    applications as well -- but somehow, having a digital chip perform
    complicated calculations feels like making things needlessly difficult,
    when there are one-step analog processes to do the same. It's like asking
    someone across the room if they want another cup of coffee by sending then
    a text message.

    Also, I don't have the need for any digital stuff at this point -- I have a
    simple (digital) voltmeter module which takes analog input, monitoring
    several different parameters selected by a switch, and some simple analog
    circuitry surrounding it, all running on a single 12 volt supply.

    I'm also pondering the idea of charging a capacitor via a resistor, and have
    a linear rising voltage (integrator) starting at the same time; when the
    capacitor voltage has reached the sensor output voltage, a simple
    sample-and-hold of the linear voltage should provide a relatively accurate
    value for the flow rate. The only critical component here would be the
    capacitor, which must have a low temperature drift.
    :)

    Anyway, thanks to everyone for their contributions. It's quite interesting
    to see how ever more electronic design questions are answered by the single
    adage "use a controller!".

    Thanks again, best regards,

    Richard Rasker
     
  17. Fred Bartoli

    Fred Bartoli Guest

    Richard Rasker a écrit :
    OK, then instead of using your log approach, one could notice the
    extreme similarity between your sensor response Vout=c1*(1-e^(-F/c2))+1
    and an RC step response Vout=Vo*(1-e^(-t/tau)) which only differs from
    your sensor response by a 1 constant. You can easily put this to work,
    without having to bother with the inherent log amp thermal problems, and
    use feedback to recover the flow value...


    R C RC = C2
    ___ ||
    -1 >----. C1 >--|___|--+--||---.
    | | || |
    .---. | ===
    Vout >--| + |---. .--------+ GND
    '---' | | |
    .-----. |
    \+ -/ +-||
    comparator \ / ->||
    V +-||---.
    | | |
    | === |
    .--o--. GND |
    GND -|D S Q|-------------'
    | |
    Clk >---|> -| ___
    | R Q|---|___|--+-----> Flow
    '--o--' |
    ---
    ---
    |
    ===
    GND


    Note that this will be a sampled system, so you have to adapt the clock
    frequency to your wanted frequency bandwidth.
     
  18. Yes, I just figured out something along the same lines, and I was just about
    to design a schematic -- but yours is quite a bit simpler and more elegant
    than what I had in mind :)
    And as your schematic bears a great resemblance to the innards of the humble
    555, it wouldn't surprise me if I could use that device for this particular
    purpose.

    Thanks for this very neat suggestion!

    Richard Rasker
     
  19. Fred Bartoli

    Fred Bartoli Guest

    Richard Rasker a écrit :
    Yes, maybe you could use the 555...
    You have access to the upper divider point to inject your Vout-1, but
    make sure the comparators have the low rail in their input range, which
    is not the normal working mode and make sure to use the CMOS version so
    that you don't have input bias currents and output switch VCEsat problems.


    Hmmm, even simpler...
    You could use the 555 with an external mos switch with it's source
    shifted, say 1V (guess why) and omit the constant subtraction from your
    Vout input.


    R C RC = C2
    ___ ||
    C1 >--|___|--+--||---.
    | || |
    | ===
    Vout >----------. .--------+ GND
    | | |
    .-----. |
    \+ -/ +-||
    comparator \ / ->||
    V +-||---.
    | | |
    | +1V |
    .--o--. |
    GND -|D S Q|-------------'
    | |
    Clk >---|> -| ___
    | R Q|---|___|--+-----> Flow
    '--o--' |
     

  20. Use a cubic spline.

    http://www.tinaja.com/cubic01.asp


    --
    Many thanks,

    Don Lancaster voice phone: (928)428-4073
    Synergetics 3860 West First Street Box 809 Thatcher, AZ 85552
    rss: http://www.tinaja.com/whtnu.xml email:

    Please visit my GURU's LAIR web site at http://www.tinaja.com
     
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