Tachometer 'Sensor Wire'

Discussion in 'The Projects Forum' started by h4tch, May 10, 2011.

  1. h4tch

    Thread Starter New Member

    May 10, 2011
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    Hi Folks,

    I'm currently working on a motorcycle datalogger, which, among other things, will monitor engine revs. I'm evaluating various methods of capturing input from the HT leads, but one fairly oft-used method is eluding me, theory wise. Hopefully, someone older, wiser or just better looking than me can shine a light on it.

    There are several off the shelf rev counters that obtain input from the HT leads by attaching a 'sensor wire' to the HT lead with a piece of tape (not coiled around it). There is no specific area on this sensor wire that needs to be in contact with the HT lead and I believe that any excess of this cable can be trimmed. The end of this cable connects to nothing. One popular tach that uses this method is the Scitsu (fitting instructions , autopsy).

    This 'sensor wire' appears to just be a piece of cable, but I am struggling with the mechanics of how it works. I figure the magnetic field in the HT lead is inducing a current in the sensor wire, but I don't see how that happens when the end of the sensor wire is in empty space. I see no closed circuit here.

    I've been unable to find a comprehensive answer via google, which suggests the answer is so simple, no one thinks it needs explaining. So, in the hope that there truly are no stupid questions, I come begging assistance.

    Thanks.
     
  2. wayneh

    Expert

    Sep 9, 2010
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    The wire sensor alone isn't a circuit, but when it's near the high voltage wire, a circuit is completed via the electromagnetic link. Any energy appearing in the sensor wire is a loss to the high voltage wire. Of course, the sensor doesn't need much energy and performance is not degraded. Apparently this sensor is sensitive enough to detect the pulse without being wrapped around the wire like the old kind.
     
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  3. mcgyvr

    AAC Fanatic!

    Oct 15, 2009
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    Automotive circuits/modifications are not allowed on this site for safety reasons.. Good luck.. you will have to find your answer somewhere else though as I'm sure this post will be locked/deleted soon.
     
  4. h4tch

    Thread Starter New Member

    May 10, 2011
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    Ah, I wasn't aware of that. Hopefully this thread does not breach the TOS as I'm not looking for the circuitry, just an explanation of the principles behind this particular sensor.
     
  5. h4tch

    Thread Starter New Member

    May 10, 2011
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    I'd not considered the circuit being bridged like that. It gives me another avenue to chase down.

    Thanks for the reply.
     
  6. Adjuster

    Well-Known Member

    Dec 26, 2010
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    The pulse voltage on a spark ignition HT lead is of the order of tens of kV. It would not be all that difficult to detect it from quite some distance away.

    This would most likely work by detecting the electric field with a capacitive probe, which would not require a closed loop detector wire, but something more akin to a short radio antenna.

    We may note that it is actually quite hard to reduce the electromagnetic emissions from a motorcycle to the point of generating no radio interference.
     
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  7. h4tch

    Thread Starter New Member

    May 10, 2011
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    Capacitive probe - that's probably it.

    Re: interference - definitely, my experience with these type of tachs have shown that their effectiveness can vary massively depending on what bike they are fitted to and what ignition systems they are working with.

    Thanks; my head scratching was starting to wear through my skull.
     
  8. retched

    AAC Fanatic!

    Dec 5, 2009
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    A Hall Effect sensor would be a fine option.

    It will detect the current in the ignition wire, non contact.

    But in the sense on a non-sensor-using-sensor wire.. Is there 2 wires inside the insulation?
     
  9. h4tch

    Thread Starter New Member

    May 10, 2011
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    Yeah, I think a Hall Effect sensor would be a fairly neat solution, but as this project is 50% practical 50% experimental, I plan on testing several different sensor configurations (hence this thread).

    I think Adjuster was right in his capacitance probe post. The sensor wire is just a piece of wire and the signal is sent as a result of capacitive coupling.
     
  10. wayneh

    Expert

    Sep 9, 2010
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    There may be capacitors in the circuitry, but it's an inductive pickup, not capacitive. Current fluctuation in one conductor induces an offsetting current in a nearby conductor.
     
  11. hewittp

    New Member

    Aug 14, 2012
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    Back from the dead because It needed to be answered.

    If the wire is wrapped around the lead with one end loose it is capacitive coupled and will create a common mode voltage in your coil (both ends the same voltage). This is because the magnetic field created by the current flowing through your plug lead curls around the lead according to the right hand rule and is parrallel to the wire in your coils around it, there is then no magnetically induced voltage in the coil.

    Now, if your wire is laid parallel to the lead it will be inductively coupled because now the magnetic lines of flux from your plug lead pass through your wire perpendicularly. The magnetic field will now induce a differential voltage across the ends of your wire and if it were a closed circuit would cause current to flow.

    Neither can ever be perfectly inductive or capacitive though.
     
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