Looking to limit/cap a frequency.

Discussion in 'The Projects Forum' started by Great Scott, Nov 2, 2011.

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  1. Great Scott

    Thread Starter New Member

    Nov 2, 2011
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    Hello. I am a novice/intermediate at electronics. I have searched and read like crazy and I have come up with a myriad of possible solutions to my simple problem.

    The problem being I have a sensor that outputs a frequency based on it's undisclosed input. The ecu responsible for this machine will throw trouble faults if the frequency gets too high and resorts to proprietary redundant strategies that result in undesirable and unpredictable operation of said machine.

    I am simply looking for a way to interrupt the sensor's output (500hz-11,000hz) and cap or limit it's perceived maximum allowable frequency (to 6,000-8,000 hz) without interfering with the signal. The signal must not stop but continue along at the newly capped frequency.

    In my readings, it would be analogous to a 0-5v automotive mass air flow clamp, where one would simply place a zener diode of the appropriate voltage between the signal and ground.

    I could use a frequency-to-voltage converter, then clamp the voltage and then run it through a voltage-to-frequency converter. That seems really redundant and fraught with error.

    I think I could use a Schmitt trigger 555/556 but I'm not sure how to model the output to my needs.

    I could program the whole mess into a propeller or stamp chip.

    I even tore apart a DSL line filter and did some testing.

    I looked into audio notch filters, but they seemed to null out the frequency instead of simply hold it.

    I feel there's an elegant and simple solution but I cant see the forest for the trees, as my mentor's used to say.

    Thanks in advance for the nudge in the right direction.
     
  2. praondevou

    AAC Fanatic!

    Jul 9, 2011
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    I would invest in the F/V --> V/F idea. It seems to be the most obvious solution. There are several IC's that come into mind, like for example the TC9400. But there are others.

    I'm quite sure that a PLL cip like the 4046 could also be used, but I just had a look at it's datasheet, it requires quite some reading and learning before you get this to work.
     
  3. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
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    A stamp or propeller do not operate fast enough to make a digital filter.

    Have you thought about a 12dB/octave low pass filter, comprised of resistors, capacitors, and inductors for steepest rolloff. See Vol II AC in the books above for examples of analog filters.

    You could use the filtered frequency for your application, while taking the non filtered signal to send to whatever is upstream.

    May I ask the purpose of such a device?
     
  4. THE_RB

    AAC Fanatic!

    Feb 11, 2008
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    Sounds like a standard application for a device to remove the top speed limit on a fuel injected car or motorcycle. There are commercial devices you can buy.

    I think it is a banned topic here too, not just being automotive but also being related to safety limits etc in the vehicle.
     
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  5. Great Scott

    Thread Starter New Member

    Nov 2, 2011
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    Interesting. I'm having a bit of difficulty visualizing what that would do to the wave form at the desired limit.

    As far as the design of the circuit, the desired response would be to limit the frequency as if the sensor had not received a continuous increase in input. So say we set our cap at 6Khz the circuit output must be reasonably transparent up to 6Khz, then must hold the frequency at 6Khz, even though the input frequency has risen above to 10Khz or 11Khz. I'm not concerned with the output as long as it can keep it in bounds.

    I must be vague about the machine for now but I can do some testing and provide accurate numerical data if needed.
     
  6. SgtWookie

    Expert

    Jul 17, 2007
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    I concur.

    As an automotive modification topic, this thread cannot continue due to the Terms of Service on this Board; in particular paragraph 6 which states:

    A Moderator will soon be along to close up shop here.
     
  7. Great Scott

    Thread Starter New Member

    Nov 2, 2011
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    It's not related to anything automotive, or even motive in general, I assure you. I've read and am abiding by your forum rules.
     
  8. Great Scott

    Thread Starter New Member

    Nov 2, 2011
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    As incriminating as it sounds, you've given me a new angle for a solution for my problem. Ironically it would run almost parallel with the procedure for defeating a speed limiter, albeit in a different frequency range.

    I'm off to the lord of Google...

    Edit: Bah. Those limiter "defecers" just divide the signal by 2. Completely unusable to me. I'm settling on the F/V --> V/F idea.

    Thanks for the help.
     
    Last edited: Nov 3, 2011
  9. Wendy

    Moderator

    Mar 24, 2008
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    Very well, what is it then? There are many members who know this subject much better than I, but on the subject of automotive mods the rules here are very clear. There are many other forums that do not have this limitation, but this site does and it is enforced.
     
  10. THE_RB

    AAC Fanatic!

    Feb 11, 2008
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    I apologise GreatScott if I mistakenly though this was for vehicle use. I ASSumed (ahem) based on your mention of the "ECU throwing a fault" and on the kHz freq which is right in line for modern speedo signals.

    If you explain more about what the machine is, and the specific type of control system (even what the problem is) people might be able to offer good suggestions. :)
     
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