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Tachometer Noise

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ohioip(at)yahoo.com
Guest

Posted: Sat Aug 23, 2008 8:24 pm Post subject: Tachometer Noise

I have a Jabiru 2200 engine and a VDO tachometer. I'm coupling one alternator lead to the tacho signal input.  It works fine at lower rpms, but at about 2000 RPM it becomes very erratic. I searched around the internet and found some discussion of the problem as being noise related with general suggestions to install noise filters, series resistors, etc. to correct the problem but few details on the circuit configuration. The most detailed info I saw described a 1 uF coupling capacitor between the alternator and the tach with a 10K resistor and a clamping diode connected to ground on the tach side of the capacitor.  Has anyone on the list tackled this problem?   



Thanks,



Mike

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jetboy

Joined: 22 Jul 2006
Posts: 233

Posted: Sat Aug 23, 2008 9:33 pm Post subject: Re: Tachometer Noise

The earlier Jabirus used a magnetic pickup at the ringear driving a VDO tach / digital hourmeter and they later changed to the same pickup placed near metal tabs on the flywheel (and different rpm calibration setting for this) This is the normal way for larger diesel engines and the like.



You are likely to have trouble using the alternator wires as source because none of them connect to ground and the regulator will cause extra pulses on the lines as it does its job. I do connect across the windings and use a digital frequency counter for calibration of my other system, and that works OK



Ralph

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_________________
Ralph - CH701 / 2200a

nuckolls.bob(at)cox.net
Guest

Posted: Sun Aug 24, 2008 6:22 am Post subject: Tachometer Noise

At 10:33 PM 8/23/2008 -0700, you wrote:

 	  Quote: 	
		  



The earlier Jabirus used a magnetic pickup at the ringear driving a VDO 

tach / digital hourmeter and they later changed to the same pickup placed 

near metal tabs on the flywheel (and different rpm calibration setting for 

this) This is the normal way for larger diesel engines and the like.



You are likely to have trouble using the alternator wires as source 

because none of them connect to ground and the regulator will cause extra 

pulses on the lines as it does its job. I do connect across the windings 

and use a digital frequency counter for calibration of my other system, 

and that works OK



Ralph


	


    An excellent response Ralph. I've always been a bit

    mystified by some system designer's decisions to attempt

    a stable deduction of alternator fundamental frequencies

    when combined with a horrid mix of strong components of

    other noises! Then one must deal with belt/gearing

    ratios to scale the resulting signal to some number that

    represents real crankshaft RPM.



    The next worse signal source is magneto p-leads, but

    we've learned to deal with those to a reasonably

    successful degree.



    Mike, there ARE some things that could be tried

    which are something of a shot-in-the-dark. Without

    doing a lucid examination of the signals the tachometer

    is attempting to resolve, it's difficult to offer

    you a high-probability "fix".



    The absolutely best signal for tachometer success

    comes from some form of dedicated sensor. Magnetic,

    optical, closed contacts, etc. etc. But of

    course, these need to be added to an existing engine

    and one would really LIKE to get say 20 pulses per

    engine revolution for the easiest crafting of

    supporting electronics.



    The instability might be a function of levels, or

    as Ralph mentioned, the effects of extraneous

    stimulus from the voltage regulator.



    The fact that your instability happens at the higher

    RPMs suggest these causes as good candidates. If

    I were to take a 95% probability shot as suggesting

    some signal conditioning, I'd go for a band-pass

    filter (op amp and hand-full of jelly-bean parts)

    combined with a squaring section (over-driven op

    amp) as a means by which undesirable signals can

    be filtered out from the signal-of-interest.



    However, it MAY be that simple combinations of

    resistors, capacitors and perhaps some zener

    diodes would make your present combination useable

    using some parts from Radio Shack.



    Question: How much $time$ do you want or are

    willing to spend? We KNOW that your present

    system can be made to function as needed . . . but

    it MIGHT evolve into quite a 'science project'.

    If you're interested in expanding your knowledge

    base . . . particularly with a goal of sharing

    your discoveries with others and don't mind the

    $time$, I'm sure that I and others her on the

    List would be glad to give you suggestions.



    On the other hand, if you're driving toward

    first-light-under-the-wheels, perhaps a more

    conservative approach is indicated . . . an

    alternate pick-off scheme would be quick and

    a sure bet.



    Continue to search the web for situations where

    others have encountered the same problem and

    have discovered a 'fix'. If you don't get some

    encouragement from that effort, make the choice

    between doing a low-risk change of technologies

    -OR- exploring signal conditioning alternatives

    with the present setup.



    Are you an electron-chaser of any degree? Do

    you own or have access to a bench power supply,

    and perhaps a 'scope?  Do you know from the

    VDO data what kind of signal the instrument

    expects to see? Since this system runs from a

    undetermined belt or gear ratio, does the

    instrument have some form of calibration

    pot? We'll need to know a lot more about the

    instrument itself to narrow down the range

    of techniques that could be applied to the

    problem. What's the VDO part number? Perhaps

    I or someone else can discover some useful

    things about this critter.



    One thing that would help is for me (or

    perhaps Ralph) to put our hands on the instrument.

    A few minutes on the bench would tell us what

    kind of pristine signal the tach likes to see

    and would put some bounds on the approach to

    crafting a useful filter. There's NOTHING

    that replaces NUMBERS to design to.



    I (and perhaps others on the List) have an interest

    in deducing a filter that would make your system work.

    But we could spend a lot of YOUR $time$ to make

    that happen and we're 100.0% sure of success. Only

    you are in a position to make the well considered choice.

    Bob . . .

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ohioip(at)yahoo.com
Guest

Posted: Mon Aug 25, 2008 6:13 am Post subject: Tachometer Noise

Ralph & Bob-



Thanks much for your excellent advice.  My motivation for asking came from the fact that both Jabiru and VDO documentation discuss using the alternator as a signal source.  After experiencing the noise problem I surfed around the internet and found comments to the effect that others had used a filter to stabilize the system, but no one provided details of the filter configuration.  Given the number of Jabiru installations out there I expected to find a relatively easy, ready-made solution.



Bob, in answer to your question, I am an electrical/electronics type and worked in aerospace for many years, so on one hand I am intrigued by the problem and am tempted to design a solution.  But, at the end of the day, I think the best, most reliable and most elegant solution is to just switch to a dedicated sensor as you and Ralph suggest. 



Thanks again,



Mike

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nuckolls.bob(at)cox.net
Guest

Posted: Mon Aug 25, 2008 7:20 am Post subject: Tachometer Noise

At 07:09 AM 8/25/2008 -0700, you wrote:

 	  Quote: 	
		  



Ralph & Bob-



Thanks much for your excellent advice.  My motivation for asking came from 

the fact that both Jabiru and VDO documentation discuss using the 

alternator as a signal source.  After experiencing the noise problem I 

surfed around the internet and found comments to the effect that others 

had used a filter to stabilize the system, but no one provided details of 

the filter configuration.  Given the number of Jabiru installations out 

there I expected to find a relatively easy, ready-made solution.


	


   Agreed.

 	  Quote: 	
		  Bob, in answer to your question, I am an electrical/electronics type and 

worked in aerospace for many years, so on one hand I am intrigued by the 

problem and am tempted to design a solution.


	


   These things are always fun if not practical . . . but

   then education was NEVER inexpensive. I presume this is

   an analog tach . . . but without a cal-adjustment,

   I'm mystified as to how calibration is accomplished.

   If it's digital, then the HARD part is a multiply-by-

   K-divide-by-N circuitry needed to scale the sensed

   signal frequency to something that displays true

   engine RPM. I'd be interested in seeing what folks

   are using to make this happen.



   In the analog world, we used to drive a transistor

   with the distributor's ignition points signal

   to get a pretty clean square wave at the collector.

   A small capacitor and two diodes differentiate the

   the edges of the square wave and depended on the

   meter's inertia to integrate the frequency. A potentiometer

   provided a means to calibrate the reading to display

   RPM. I built a bucket full of these things for

   the hot-rod guys using surplus meter movements.



 	  Quote: 	
		  But, at the end of the day, I think the best, most reliable and most 

elegant solution is to just switch to a dedicated sensor as you and Ralph 

suggest.


	


   Yeah, life IS that way sometimes. What does this

   engine use for ignition triggering?



   Bob . . .

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