Note on the pdf document showing the thermal MAF sensor output.

The sensor output voltage vs actual mass air flow is nonlinear and not a
simple function.  It as a complex polynomial and can only be reasonably
dealt with by a lookup table using a uC as the controller.

This effectively makes a simple analog-555 approach impossible.
Craig Taylor


Unfortunately I have to agree but we have come up with a custom $5 555 that
solves the problem. It consist of an A to D followed with a look up table
followed by a PWM generator all for $2.50 less programming and shipping:)

Paul Lamar

Craig replies;

I think that is the best approach.

Craig Taylor



Is the mass flow vs. voltage chart real data (taken on an actual sensor)?

I plotted it and then did a curve fit (two ways).  The curve fits
reasonably well to an exponential regression..  I would think it
possible to come up with an analog circuit that follows the curve
reasonably well.  I believe the gain curve of many transistors (around
Vt) is somewhat exponential..

Then if needed, add some piece-wise comparator based tweaking to the
pulse width - above a certain flow voltage, adjust the fuel up (or
down) by a certain amount.

I'm just saying, don't get scared into digital by the non-linear
response of the MAF.  This is non-linear, but not in terrible way..
If you go to digital (a lookup table), do so for other reasons.


Matt-


I don't know but it matters not now as we can adjust the curve
based on dyno engine test using the look up table. It is much
simpler than it seems with perhaps one page of code.
Read the MAF voltage. Use the number to go into the look up table. Read the value in the table and send it to the PW generator. Start over.

The program code won't change from chip to chip. Only the values in the table.

With a ten bit A to D you have one part in 256 part resolution or
.02 volts. If the mixture range is 10:1 to 20:1 the resolution on mixture setting is .04. In other words 15:1 or 15.04 :1.


Paul Lamar


Matt, Paul

Matt you make a good point.  The results of a simple analog "linearizing" circuit won't be exact but could be within 5% or so without too much strain, I think.

One suggestion is to use a rate multiplier to produce an X**2 (X squared) result.  That could be squished around to be an "X to the 2.xx" by changing the gain of the input to one of the multiplier ports.
I just might try to simulate that approach.  Of course, you can use a CMOS 555 as a rate multiplier.  Handy little chip :-)

Craig Taylor