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p-lead signal conditioning or tach gen usage

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nuckolls.bob(at)aeroelect
Guest

Posted: Thu Oct 22, 2009 12:00 pm Post subject: p-lead signal conditioning or tach gen usage

At 09:44 AM 10/22/2009, you wrote:

 	  Quote: 	
		  

<capav8r(at)gmail.com>



Bob:



My application is a russian radial engine and I am trying to use the 

tach input of an EFIS system in the aircraft.  The EFIS expects a 

nice square wave from a Hall Effect sensor.  However, I cannot mount 

a Hall effect sensor on the Russian mags as there is no port to do 

so and even if there were, the threads would be metric pitch.



The tach gen is a three phase AC generator.  The connector on the 

tach gen is three pins so it is likely Y-wound and uses the case as 

neutral.  According to specs on the engine, the tach gen turns at 

half of the engine rpm.  What I see is the following:



1) I can use a single phase of the tach gen as you proposed but I 

will then only get one-half pulse per revolution of the engine since 

the gen turns at half the engine rpm.  I was thinking of doing full 

wave rectification of the signal to get back to one pulse per revolution.


	


   This may not be true. It depends on how many poles are in

   the rotor magnet and stator windings. I think the machines

   we were building had 4 poles.



 	  Quote: 	
		  2) I could condition the p-lead signal and tell the EFIS that there 

are 4.5 pulses per revolution.  However, I don't know how to 

condition the signal properly.  This method would provide no engine 

RPM during a mag check but I'd be OK with that.



We are putting a scope on one phase of the tach gen output and the 

p-leads tomorrow.  I'll take a picture of the scope as this will help.


	


   Okay. That's a good start. Also check continuity

   between any one signal lead and case . . . I suspect

   you'll find that there is no "neutral" connection.



   If you do a full-wave rectification of the 3-phase

   output, you will indeed get a ripple signature that

   is 3x the frequency of a single phase . . . but it

   drops to 5% pk-pk of average. Now you'll need to process

   that small signal that becomes even smaller at idle and

   perhaps run-up rpms.



   How hard is it to get your hands on another

   tach generator . . . it doesn't need to be

   compatible with your tach, only fit the engine.

   I can probably show you how to build a many-pulses

   hall (or perhaps a bunches of pulses electro-

   magnetic) transducer into the old tach generator

   case.



 	  Quote: 	
		  PS - bought your book and it is excellent.  Thanks for that and all 

your help to us with experimental aircraft!


	


   Thanks for the kind words. I'm pleased that you

   find the work useful!

        Bob . . .



         ---------------------------------------

        ( . . .  a long habit of not thinking   )

        ( a thing wrong, gives it a superficial )

        ( appearance of being right . . .       )

        (                                       )

        (                  -Thomas Paine 1776-  )

         ---------------------------------------

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ChangDriver

Joined: 15 Sep 2007
Posts: 266

Posted: Thu Oct 22, 2009 12:56 pm Post subject: Re: p-lead signal conditioning or tach gen usage

Bob:



I think I know where you are going with the tach gen.  We could tap the side of the case and put a hall effect on it to "watch" the rotor magnets go by.  I suspect the stator windings would need to be removed.  Am I on track?  



To answer your question, yes, I can get another tach gen for the engine with no problems.  About $150 for a new one...a little less for a used one.  The Hall Effect can be obtained fro Aircraft Spruce.



I'll post pictures of the o-scope and also look at the terminal to case resistance to see if there is any connection.



Craig

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ChangDriver

Joined: 15 Sep 2007
Posts: 266

Posted: Thu Oct 22, 2009 5:22 pm Post subject: Re: p-lead signal conditioning or tach gen usage

 	  Quote: 	
		  If you do a full-wave rectification of the 3-phase

output, you will indeed get a ripple signature that

is 3x the frequency of a single phase . . . but it

drops to 5% pk-pk of average. Now you'll need to process

that small signal that becomes even smaller at idle and

perhaps run-up rpms. 	




Bob, I was thinking of only rectifying one phase not all three.  My thought was that the tach gen is rotating at half engine speed, I'd get another "peak" in the signal from one phase and essentially double the frequency.  Craig

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ChangDriver

Joined: 15 Sep 2007
Posts: 266

Posted: Sun Nov 01, 2009 2:59 pm Post subject: Re: p-lead signal conditioning or tach gen usage

Bob:



Put a scope on the tach gen output.  The situation is the signal from the tach gen goes to a 5.1 V zener diode, then to a comparator against a reference voltage to generate a square wave.



Attached is the picture of the waveform at low rpm (approx 800).  Note the spikes in the wave.  Also, the voltage is near the zener voltage so the zener never gets driven into breakdown so that is why the unit has a flakey rpm reading at low rpm.



So, would the waveform squaring circuit you posted in an earlier post work to square this waveform?  If so, I have 28 VDC available in the aircraft.  I'd need to use a 28-5 V converter chip to get the 5 VDC on the collector as you drew it.  Any suggestions for this?



Craig

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Tach Gen 800 RPM.JPG

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ChangDriver

Joined: 15 Sep 2007
Posts: 266

Posted: Sun Nov 01, 2009 4:01 pm Post subject: Re: p-lead signal conditioning or tach gen usage

Bob:



Forgot to mention that at full RPM (2400) the tach gen signal is a bit more than 40 v peak to peak.



Craig

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