Pressure Transducer help

Discussion in 'The Projects Forum' started by electratech, Jul 22, 2012.

  1. electratech

    Thread Starter Member

    Jan 5, 2009
    21
    9
    public post for help resulted deleated parts stock
     
    Last edited: Jul 26, 2012
  2. #12

    Expert

    Nov 30, 2010
    16,337
    6,820
    I'd just use the vertical position knob on my scope but you probably have one of those newer digital storage scopes that make measurements so much easier.
     
  3. electratech

    Thread Starter Member

    Jan 5, 2009
    21
    9
  4. JohnInTX

    Moderator

    Jun 26, 2012
    2,347
    1,029
    Not as clever as #12 but if the scope has a Channel 1 - Channel 2 function (hard to tell from the manual), you could put your .5Vmin (scabbed off the transducer supply?) on channel 2 and the scope would do the offset. Ugly but would work.

    You couldn't just put a pot on the output since that would also affect the effective slope (gain) of the transducer and change its calibration (V/PSI). If the gain is calibrated to the scope already, you're stuck with adding a -.5V offset one way or another.
     
  5. cork_ie

    Member

    Oct 8, 2011
    348
    58
    I don't know if you already have bought your transducer or not but I bought one of these on ebay 2 years ago and it works superbly with PicoScope which is similar to your GoScope. They appear to be very high quality and I can highly recommend them.

    http://www.ebay.ie/itm/Pressure-Tra...t=LH_DefaultDomain_3&var=&hash=item92b041d066

    Specs are a little different to yours as the lowest value they still have in stock is 60Bar 870PSI
     
  6. electratech

    Thread Starter Member

    Jan 5, 2009
    21
    9
    Thanks Guys, This is a project that is a little over my head but worth the brain strain if I can get something to work.
    They already exist on the market but because it's "automotive" they gouge us.

    I just cant afford to keep spending and spending, I have made alot of my own tools, it pays off to know a little about electronics but I tell you what, it's not easy and I take off my hat to you guys. I wish I had half the brain power as some of you guys.
     
  7. electratech

    Thread Starter Member

    Jan 5, 2009
    21
    9
    hey JohnInTX

    Is that a neg .5v?
    think an in326 would do well for setting range or to "zero"?

    scope is set as 0v=0psi , .1v=12.5psi, tenth increments all the way up to 4.0v=500psi

    All I want to do is get a pressure transducer in a cylinder of an engine and measure signal integrity, getting it close as possible to accuracy as I can but more importantly getting a good signal displayed to interpret the pattern.
    The other channle is for primary ignition signal for cam and crank timing analysis.
    so need the other channle for sure.

    Any guideance, thoughts, ideas are welcome for sure. Thanks!
     
  8. JohnInTX

    Moderator

    Jun 26, 2012
    2,347
    1,029
    If you used both channels of the scope, it would be a +.5v since the scope in A-B mode would do the subtraction i.e. whatever was on channel A (the transducer) would have a +.5V subtracted from it but yes, you get the idea, you want to lop off .5V from your signal without affecting the gain (Volts/PSI) calibration of the transducer.

    If you put a diode in series with the signal, you would reduce the signal by the forward voltage of the diode. Unfortunately, that voltage will vary depending on the current through the diode, from diode to diode and the same diode depending on temperature. Not recommended here.

    Basically, you are describing a signal conditioning problem. Your transducer happens to have a gain that is calibrated to the scale that you are looking for but its offset is wrong. The preferred solution is to remove/adjust that offset with some sort of summing circuit. You want its output to be input-.5V. AAC has some reference mat'l here on how to build a basic one:
    http://www.allaboutcircuits.com/vol_3/chpt_8/8.html
    More is on the following page. The 2ed diagram is a better difference amp. The 3rd page is a real IA.

    Frequently, you can get by with an op amp as shown in the ref. and I suspect you can here. An IA would be the next step. You don't need R-R if you can provide power supplies that are greater than the signal.

    For your app, I'd consider some cheapo IA. Don't have a P/N handy but check the usual suspects:
    maxim-ic.com
    national.com
    analog.com
    ti.com/analog

    Pretty much anyone building analog ICs will have some basic IAs that are easy to use.

    If it were me, I'd put a pot across a 1.25V bandgap reference with the wiper to the (-) terminal of a cheap IA and the transducer signal to the (+) signal. Adjust the pot to .5V and gain to 1 and you are in business.
     
Loading...