Lawnmower Man

dn dn at dlogtech.cuc.ab.ca
Fri Feb 16 22:59:56 GMT 1996


>   When I get it all together I'll post it on the web Page... Besides I'm going 
> to use a MPX2200, A motorola pressure sensor rated for 200KPA, with .2mV per 
> KPA, it has thermal recalibration, and as a plus It has differential input so 
> that the values stay pretty good across the spectrum. I op amp it so that It's 
> span is 2V with an offset of .25V, it's great!!!!

I've used this sensor, (although not as a MAP sensor) it works great.  If
you have the differential version, you can use it just like an absolute
sensor by reversing the inputs and applying vacuum to the pressure port, and
leaving the vacuum port open to the atmosphere.  This may also require 
reversing the inputs to your opamp to get the right direction of signal.  
(increasing vacuum, decreasing voltage).  The differential version is much 
easier to get than the absolute.  The MPX2200 has been available for some
time now, and the temp comp is nice...  Forget about trying to compensate
an MPX200 yourself, it is very difficult and not very accurate at best...

On the subject of opamps, try the AMP04 from Analog Devices.  It's a true
instrumentation amp, single supply, and almost made for the MPX series 
transducers.  This chip, and one resistor, is all you need for gains of
up to 1000.  Add another resistor to offset the bridge a little and you've 
got a .5V - 4.5V output.  Add a cap and you've got a first order lopass 
filter as well.  Full signal conditioner with 4 parts!

As far as the MAP filtering goes, I'd combine 3 methods to get a nice, 
clean signal:

1. Put an orifice and a small reservoir between the sensor and the manifold.
You'll have to experiment with orifice size to get a good compromise between 
response time and filtering.  For the reservoir, just use a small disposable
filter, such as an inline gasoline filter.  This will give you a small volume 
to dampen pressure pulses.  Place the orifice between the filter and the 
manifold, and the pressure sensor after the filter.  This gives you the 
pneumatic equivalent of an RC network, the orifice is the R, and the volume
is the C... 

2. Put some filtering in the electronics.  Set a corner frequency just below
where you have the most noise, ie at idle speed.  Just use an RC network,
or if you want to get fancy, a 2nd order lopass active filter.  This will
also take out most of the electrical interference from ignition, etc.

3. Oversample the hell out of the analog signal, and do some digital 
filtering of the data from the A/D converter.  Add up a bunch of samples 
and divide the result by the number of samples.  This will give you a nice, 
low frequency averaging to the signal.  You can easily adjust the number
of samples to get whatever averaging you want.  Cascade two averaging
routines to get a second order system...

I recently did a similar project with nearly the same type of problems you
would find in your MAP application, and the combination of the 3 types of 
filtering gave me the best results...

regards
dn

--
---------------------------------------------------------------------
 Darrell A. Norquay              Internet: dn at dlogtech.cuc.ab.ca     
 Datalog Technology Inc.         Bang: calgary!debug!dlogtech!darrell
 Calgary, Alberta, Canada        Voice: +1 (403) 243-2220            
                                 Fax:   +1 (403) 243-2872            
    @ +                                                                
     <                                                         
    __/    "Absolutum Obsoletum" - If it works, it's obsolete    
 --------------------------------------------------------------------




More information about the Diy_efi mailing list