"Invert" Sensor Signal

Discussion in 'The Projects Forum' started by GTX, Jan 6, 2007.

  1. GTX

    Thread Starter New Member

    Jan 6, 2007
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    Hi.

    I guess what I really want to do is invert the signal and add it back to the reference signal. I don't want a negative signal voltage.

    I've been trying to solve an automotive electronics adaptation problem with my rather basic knowledge of electronics. I have a dual track position sensor in which the sensor outputs move in opposite directions. i.e Sensor 1 goes from roughly 0.5V to 4.5V while Sensor 2 goes from 4.5V to 0.5V. Both sensors share common +5V and 0V reference circuits. The control module is looking for both sensors to be moving in the same direction and I would like to reduce the range to approximately 0.5 to 4.0V. I will have a spare +5V and a spare 0V reference circuit from the module. From my research, I expect that it would be possible to turn the Sensor 2 signal around using an op amp circuit. I have a couple of LM358 op amps. http://www.national.com/pf/LM/LM358.html Not sure that I have it quite right though. So I am back to square one. Should I be able to do this with what I have available under the hood(i.e. no -ve supply)?

    If I can get both signals moving in unison(some slight variation is allowed), then I will just have to keep the diagnostics satisfied that the sensor circuits are not shorted. I believe that one of the 5V ref circuits is periodically grounded to verify this in designs where both sensors' signals move in the same direction.

    Any help or advice is much appreciated.

    Thanks.
     
  2. beenthere

    Retired Moderator

    Apr 20, 2004
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    Hi,

    Ordinarilly, using a couple of op amps to invert a signal and to reduce the magnitude would be no problem. Things under the hood of a car are pretty awful, though. You would be advised to find components with a military spec on them, in order to get amps that can handle the temperature variance under there. Just about any op amp that will go rail-to-rail should do. This is significant, because you only have a 5 volt supply, and most op amps won't swing an output to more than 1.5 volts of the supply voltage (the LM358, for instance). A proper inversion will require a negative 5 volt supply, though, but a charge pump voltage inverter can handle that.

    Out of curiousity, why do you need to do this? Is this sensor something you are trying to add as part of a project? If it is part of the original equipment, have you thought about how the car's computer is going to handle the new signal? These sensors track a position. There will be zero difference at the center, and 4.5 volts at either extreme as things are now. Why can't the control module deal with the two signals as they are?
     
  3. GTX

    Thread Starter New Member

    Jan 6, 2007
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    Thanks for your input. Ambient temps were a concern as well. I guess I could also run a 12-14V supply if needed. Just trying to keep it as simple as possible. I have been researching voltage inverters today as well.

    This for a performance application where a larger non OE throttle body(TB) from a different vehicle is utilized. The sensor is integral to the TB and intended to be non-serviceable. The larger throttle body being from a newer application where the throttle actuator control module(TAC) is incorporated into the PCM rather than a separate entity. The replacement TB can be made to work using a TAC module from an older application in which the sensors' outputs move in opposite directions, but full throttle range is not acheived due to a wider sensor output range than the intended application. I have been able to open up the TB and modify/isolate the circuitry of the sensors, etc. to acheive full opening but something more plug n play/less risky is desirable. Particularly if the OE TAC module can be retained.

    Again, the OE TAC module is expecting both sensor outputs to be increasing as the throttle opens which is not the case with the replacement TB. The TAC module appears to convert the sensor output to a % or voltage value that is reported to the PCM. The PCM does not seem to be concerned with the direction of the sensor outputs themselves as it has worked with both TAC module/TB combinations.

    Thanks again
     
  4. beenthere

    Retired Moderator

    Apr 20, 2004
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    Hi,

    One possible dodge about the signal is to ignore one, and strap the one signal that moves in the desired direction so it appears as both. that way,the only signal massaging left is to reduce the signal at the positive endpoint. that means only one op amp to use.
     
  5. GTX

    Thread Starter New Member

    Jan 6, 2007
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    I have attempted something like that. But I knew even less at that point. So it may have been a little crude. I was able to get past the key-on tests but usually upset the TAC module by 30-50% throttle. Could be that I was tripped up by the TAC module diagnostics or perhaps I just exceeded the allowed variance between the signals(I don't think that was the case according to my logs of the signals though). Apparently, in this type of design(sensor signals in sync), the TAC module will cut one of the two +5V ref circuits periodically to ensure that it is indeed seeing 2 separate sensor output signals. Not sure if the test runs only at key on or while the engine is running as well. Might try logging a stock vehicle to see if there are any spikes in the signals while driving. I guess it can be made to work if it is always the same circuit that is cut.
     
  6. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    Hi,

    If this is only something like a throttle plate sensor, having two signals seems a bit like overkill. I really can't imagine why the computer would dump power to a sensor to be sure it was attached. If the signal is within range, it is entirely reasonable to assume the sensor is there.

    Probably be a good idea to see what a stock setup looks like. Life was easier before computer engine controls.
     
  7. GTX

    Thread Starter New Member

    Jan 6, 2007
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    This is a drive by wire(electronic throttle control/ETC) setup so I guess maintaining some redundancy is important.

    That said, I did some logging on another vehicle(same OE TB and TAC module) tonight and didn't see any evidence of this. Although I have read about it being used as a strategy to ensure that the sensor circuits aren't shorted where the signals move in the same direction. Throttle position is also compared to calculated and measured airflow, RPM, etc., so there are plenty of sanity checks. Anything out of line with the tables in the calibration triggers Reduced Engine Power(limp) mode which won't even get you up a hill, so having a secondary sensor signal doesn't do much for you anyway.

    Maybe this can still be accomplished as you have suggested.
     
  8. GTX

    Thread Starter New Member

    Jan 6, 2007
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    Well, I was successful in cloning the 0>5V signal with a voltage follower and managed to trick the TAC module into believing that there are 2 completely separate circuits. The 5V reference to the 2nd circuit does indeed continuously cycle between ground and 5V unless a fault is detected in the primary sensor circuit.

    This setup is working nicely and I have full throttle range. But I would still like to try using the actual signals from both sensors. This will provide additional protection in the event of a voltage drop in the 5V ref circuit(loss of correlation between the 2 sensor signals). I will need +/- supplies for my op amps. This can be derived from either the vehicle 14V system or one of the 5V reference circuits. I am trying to keep size/complexity down and maximize reliability.

    Any suggestions on the best way to derive the -ve supply(circuit/chip)? Keeping in mind that the components should be rated for -40 to +125C. I have found some potential solutions that I may use for testing only, but they do not meet this requirement.

    Thanks.
     
  9. GTX

    Thread Starter New Member

    Jan 6, 2007
    7
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  10. GTX

    Thread Starter New Member

    Jan 6, 2007
    7
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    edit: Question removed. Problem was a wiring, rather than circuit design, issue. Circuit is working well. :)
     
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