DIY-WB Output Voltage Scaling

[Len Sabatine]

     I believe your cautions here are well placed. Entering into
     WB territory could be stepping into a very large can of worms,
    depending upon what an individual is going to use the Diy-WB
    tool for. In the hands of experienced tuners , as a monitor,  is ,
    a basic need. Using this tool to interact directly with and modify
    a PCM's expected NB routines is another matter . These operations
    need to be carefully tested and developed, IMO.
    The age old theory vs practice is especially important in this scenario.
    Len

>Now that the WB project is off the ground, a potentially
>even bigger problem looms: How is it best used?  Here
>are some thoughts about that.
>
>Say I'm changing fuel injector pulse width (changing the
>fuel added to a volume of air).  Lets say I make a 1/10 ms
>change.  I would expect the output format of my WB unit
>to change a certain amount.  If I keep making 1/10 ms
>changes (same amount of air), I would expect the WB
>output format to keep taking equal sized steps across
>the lean to rich scale.  In other words, a linear response.
>
>I would be annoyed if the WB output showed a huge
>step in the lean area, but a small step in the rich area,
>for the same size change in injected fuel.
>
>If you agree with the above, then how might that be
>achieved?  The WB output voltage curve faithfully tracks
>the sensor, with a continuously changing slope and an
>extra bend at stoich.
>
>What about a direct A/F readout?  The very same
>amount of fuel change required to make one A/F division
>change at the rich end, will make TWO A/F division changes
>at the lean end.  Thats because A/F is NOT directly
>proportional to the change in fuel.  A/F has been a handy
>handle in the past, but just what IS the linear tuning
>function I (and perhaps you) want?
>
>I have cooked up an output display for the WB project
>which (I hope) displays a LINEAR input fuel change to
>output display change.  Just how does this work?
>
>Imagine you have this box of air, and you add a drop of
>fuel, say 1 % or 1/100th by weight.  Then more and more
>drops.  The ratio of fuel to air (by weight) is increasing
>LINEARLY with the number of drops.  Just plot 0/100,
>1/100, 2/100, 3/100, 4/100 against 0, 1, 2, 3, 4 etc drops,
>and you get a nice straight line.
>
>So what about A/F?  WAIT A MINUTE, the above is a
>description of FUEL/AIR, NOT AIR/FUEL.  F/A is the
>inverse of A/F.  OK, lets plot the inverse series,
>100/0, 100/1, 100/2, 100/3, 100/4, 100/5  etc against
>the number of drops of fuel.  Gee, the first number is
>infinity, then 100, then 50, then 33 1/3, 25, etc.  This
>has a TERRIBLE curve to it, not linear at all.  When
>we get into the practical range of 100/5 to 100/10, the
>curve is still pretty bad.
>
>So from the perspective of  tuning aid, what is the
>answer?  Here is my proposal.  I have this moving
>bar graph of LEDs which is perfectly linear, as
>described above.  Along it is a scale of A/F.  Since
>A/F is NOT linear, the A/F divisions are twice as far
>apart at the rich end, compared to the lean end.  We
>have linearity for tuning AND an A/F readout.  Oh
>yea, the sensor has a different level of output on
>each side of stoich.  That's been taken care of too.
>
>Is this a good solution?  I'm asking you.  The first
>display is already operational at my house.  A
>couple of you already have a picture; sorry, I don't
>have a scanner.  The sensor limits are not centered
>around stoich, so the scale is lopsided.  If we use
>a practical operating range of 19:1 to 10:1, the rich
>end from stoich will be twice as long as the lean
>end.  OR, we could make the scale twice as
>responsive at the lean end to equalize the length,
>but forget about linear tuning response from end to
>end.  Your thoughts please.  This display does not
>prevent you from attaching any other output format
>devices you like.
>
>For those interested in the nitty gritty details of this
>display device, write me and I'll return the preliminary
>(lengthy) writeup.  If you would like to get a better
>feel for this, get out that WB output voltage vs A/F
>curve.  Calculate the inverse of A/F (divide 1 by the
>A/F), and plot these numbers (F/A) against the
>voltage.  You will get a straight line, with a bend at S.
>The sensor is LINEAR after all, if we could fix that
>bend at stoich.  The display design fixes that too.
>
>Bruce Roe
>
>
>
>On Fri, 24 Aug 2001 19:43:50 -0500 jll at edge.net (John Lamb) writes:
> > Are you saying that the Display has an embeded PIC that allows you to
> > program a segmented curve fit?
> > That's what is needed here to linearize the output.
> > Another method is to use the an A/D to address memory locations that
> > linearize the output.
> > Sorry if this is old discussion.
> >
> > JL
> >
> > Peter Gargano wrote:
> > >
> > > It should be pointed out that the current incantation of the
> > > DIY-WB unit has a non-linear voltage:AFR transfer function.
> > > The actual transfer function is described here:
> > >
> > >   http://www.diy-efi.org/diy_efi/projects/diy_wb/
> > >
> > > See "Output voltage/AFR reference". A graphed version is here:
> > >
> > >   ftp://ftp.diy-efi.org/incoming/DIY_WBafrVolts.gif
> > >
> > > The curve can be thought of as a series of straight lines
> > > for simple interpolation, but it means that a simple digital
> > > panel meter connected to the WB output with a simple scaling
> > > circuit will not be able to show accurately a large range of
> > > AFR ratios. This is why I suggested the Jaycar kit/Silicon Chip
> > > kit with 16F84 PIC and 3 digit display that comes with source
> > > code that could be modified to scale the DIY-WB's output to the
> > > above transfer function.
> > >
> > > If you're logging the DIY-WB's output and post-processing the
> > > logged voltage, then this isn't too much of a problem.
> > >
> > > For the DIY list, here's the information I posted to GMECM
> > > yesterday.
> > >
> > > Product Name:        DIGITAL FUEL MIXTURE DISPLAY KIT
> > > Catalogue Number:    KC5300
> > > Price:               AU$62.95  (less than US$35) + freight
> > >
> > >   http://www.jaycar.com.au/home.htm
> > >
> > > Peter.
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