Checking Tach

This forum has been developed to discuss maintenance topics in Canada.

Moderators: ahramin, sky's the limit, sepia, Sulako, North Shore

Post Reply
Message
Author
NeverBlue
Rank 8
Rank 8
Posts: 907
Joined: Mon Feb 04, 2008 12:53 pm

Re: Checking Tach

#126 Post by NeverBlue » Tue Dec 29, 2015 7:41 pm

Ahramin,
Your the one that pointed out the xtal accuracy in the first place...I told you it meant nothing...

Now you agree?
---------- ADS -----------
  

photofly
Top Poster
Top Poster
Posts: 6583
Joined: Tue Jan 18, 2011 4:47 pm
Location: Making aviation exhausting, everywhere

Re: Checking Tach

#127 Post by photofly » Tue Dec 29, 2015 7:48 pm

Er, no. Exactly the opposite.

You insisted the calibration of the tool was everything. Remember your skepticism about my Fluke meaning the powerline frequency correctly and how that uncertainty (I hadn't checked my meter at 400Hz and 10kHz) made the whole tool worthless?
---------- ADS -----------
  
“This isn’t flying, it’s falling. With style.”

User avatar
Pat Richard
Rank 8
Rank 8
Posts: 752
Joined: Sat Feb 28, 2004 10:36 pm
Location: all over

Re: Checking Tach

#128 Post by Pat Richard » Tue Dec 29, 2015 8:14 pm

Photofly - thanks for sharing your project and demonstrating/exercising a level of patience(more like tolerance) that is worthy of a special ed instructor. I tip my hat to you.
Im not an M1 guy, but I've been finding your thread very interesting. Hope your product does well for you on the market.

Cheers'

Pat
---------- ADS -----------
  

ahramin
Rank Moderator
Rank Moderator
Posts: 5574
Joined: Tue Feb 17, 2004 5:21 pm
Location: Vancouver

Re: Checking Tach

#129 Post by ahramin » Tue Dec 29, 2015 8:34 pm

NeverBlue wrote:Ahramin,
Your the one that pointed out the xtal accuracy in the first place...I told you it meant nothing...

Now you agree?
No No No. You've failed to numerate it properly. It's the overhead scrantons that are important.
---------- ADS -----------
  

photofly
Top Poster
Top Poster
Posts: 6583
Joined: Tue Jan 18, 2011 4:47 pm
Location: Making aviation exhausting, everywhere

Re: Checking Tach

#130 Post by photofly » Tue Dec 29, 2015 8:37 pm

I wonder if you can pick up traces of the ignition timing by looking at the 12v out of the cigarette lighter socket?
---------- ADS -----------
  
“This isn’t flying, it’s falling. With style.”

cgzro
Rank (9)
Rank (9)
Posts: 1735
Joined: Wed Jul 25, 2007 7:45 am

Re: Checking Tach

#131 Post by cgzro » Tue Dec 29, 2015 9:38 pm

I wonder if you can pick up traces of the ignition timing by looking at the 12v out of the cigarette lighter socket?
Very likely, be an interesting experiment, alternatively you could put an antenna right up close to the ignition switch, or even treat the key as an antenna and look for the RF directly. Put scope leads on the key and ground and see what you get relative to the cigarette lighter?
---------- ADS -----------
  

cgzro
Rank (9)
Rank (9)
Posts: 1735
Joined: Wed Jul 25, 2007 7:45 am

Re: Checking Tach

#132 Post by cgzro » Tue Dec 29, 2015 9:55 pm

What in my posts has been negative???? Wrong??? Please.....
Just off the top of my mind...

1) Multiple optical tachs would not read the same..No- I've seen the exact opposite and at much higher RPM's for model engines.
2) That they require calibration at multiple RPM's. No - Only the original needs checking to ensure no bugs or faults. Manufactured copies only need a check at one point within the specified range and the rest is unnecessary due to linearity of the entire process.
3) That it would not work reliably due to outside light sources etc. No - They do work. I've used them for years. They have a very simple work or don't work behaviour. Others here agree and a prototype was built that works.
4) That they are voltage sensitive. No they are not.
5) That the crystal has nothing to do with accuracy. Yes It does, its the primary source of all accuracy.
6) You failed to identify any real sources of problems such as those I alluded to (segment failure, very low/high temperatures) software jitter or one that photofly indicated, the sweep error at larger distances.
7) That its better to disassemble the tach, put a drill on it etc. i.e. disassembly v.s. completely non intrusive testing.
---------- ADS -----------
  

NeverBlue
Rank 8
Rank 8
Posts: 907
Joined: Mon Feb 04, 2008 12:53 pm

Re: Checking Tach

#133 Post by NeverBlue » Tue Dec 29, 2015 11:50 pm

1) multiple optical tach testers used in many different scenarios...compared to a calibrated lab tach tester...pretty much guaranteed

2) you don't know what calibration is....you should read the definition...

3) he showed that it didn't work with 1 more bloddy light source present...look at his nice picture.

4) another brilliant statement...what about no volts...yah electronic measuring equipment is not voltage sensitive for accuracy at all...

5) the xtal has everything to do with the accuracy of the display yes...the accuracy of the clock yes...the accuracy of the math, some....but the accuracy of the entire unit?...sorry no.

6) I failed to because you say so...ok...there are a whole slew of things that can cause problems...I've only talked about a few...so what?...does ignition affect it? You don't know that it does or doesn't.

7) putting words in my mouth huh...I never ever ever ever said that was the best way...ever.....I was merely giving Ahramin an option with the equipment he said he had....maybe I should have told him instead to hold the tester 6 inches away from the spinning prop?
---------- ADS -----------
  

NeverBlue
Rank 8
Rank 8
Posts: 907
Joined: Mon Feb 04, 2008 12:53 pm

Re: Checking Tach

#134 Post by NeverBlue » Wed Dec 30, 2015 12:08 am

made the whole tool worthless? 
I never said that either...I use Flukes all the time...mine's a 77 that I bought in 1986..but I must calibrate it every year...rules...
It' is very inaccurate when the battery is low...resistance measurements especially.
The 20 series units I use very often as well...their frequency counters are garbage at higher frequencies and their AC measurements become unreliable at high frequency voltages...thus the benchtop 8000 series we use for calibration.
I wonder if you can pick up traces of the ignition timing by looking at the 12v out of the cigarette lighter socket?
Traces yes I think...but at all different levels...and 6 per revolution at all different levels...since it's induction separation = attenuation...a new battery in the aircraft may take it all away for a bit as well.
---------- ADS -----------
  

photofly
Top Poster
Top Poster
Posts: 6583
Joined: Tue Jan 18, 2011 4:47 pm
Location: Making aviation exhausting, everywhere

Re: Checking Tach

#135 Post by photofly » Wed Dec 30, 2015 5:38 am

On the point no 1, that multiple testers give different answers: remember the saying of Confuscious.

The man with one watch knows the time. He with two is never sure.
---------- ADS -----------
  
“This isn’t flying, it’s falling. With style.”

photofly
Top Poster
Top Poster
Posts: 6583
Joined: Tue Jan 18, 2011 4:47 pm
Location: Making aviation exhausting, everywhere

Re: Checking Tach

#136 Post by photofly » Wed Dec 30, 2015 6:33 am

NeverBlue wrote: 3) he showed that it didn't work with 1 more bloddy light source present...look at his nice picture.
To be accurate, I showed I couldn't easily get a steady result by shining a laser beam off the silver tape. It was the tape and laser combo that was at fault along with the idea of reflecting light rather than interrupting the light with the prop. I haven't yet tried it by shining a laser beam through the prop.

I haven't preciously mentioned: it works with voltages from 3.5 (and higher) down to 2.43 volts (where the display goes blank) with no change in frequency.
---------- ADS -----------
  
“This isn’t flying, it’s falling. With style.”

NeverBlue
Rank 8
Rank 8
Posts: 907
Joined: Mon Feb 04, 2008 12:53 pm

Re: Checking Tach

#137 Post by NeverBlue » Wed Dec 30, 2015 7:37 am

I haven't yet tried it by shining a laser beam through the prop.
Of course that'll work better....so will a flashlight through the prop...so will any DC powered light that has no ballast through the prop...but just one and nothing else.

...So yours works great at a low voltage...great...

my Fluke doesn't...and it's LCD display will work down to millivolts...

Just one example of an LCD display device....what's the truetach?
---------- ADS -----------
  

photofly
Top Poster
Top Poster
Posts: 6583
Joined: Tue Jan 18, 2011 4:47 pm
Location: Making aviation exhausting, everywhere

Re: Checking Tach

#138 Post by photofly » Wed Dec 30, 2015 9:16 am

Tried two devices together today. They agreed within 2rpm in flight and within 5rpm during ground idle, both cycling up and down together, slightly out of phase. The variation over time of the engine rpm was about 15rpm on the ground and a few rpm in flight so the agreement between the units was better.

There was no problem holding them steady, they worked well next to each other sitting still on the glare shield.
---------- ADS -----------
  
“This isn’t flying, it’s falling. With style.”

NeverBlue
Rank 8
Rank 8
Posts: 907
Joined: Mon Feb 04, 2008 12:53 pm

Re: Checking Tach

#139 Post by NeverBlue » Wed Dec 30, 2015 11:27 am

...is it even close to the xtal accuracy?

So the same exact situation for two exact same units and they DON'T read exactly the same...why not?...you can't even answer that.


How much you want to bet our four 1790's do read exactly the same...because they've all been calibrated.
---------- ADS -----------
  

photofly
Top Poster
Top Poster
Posts: 6583
Joined: Tue Jan 18, 2011 4:47 pm
Location: Making aviation exhausting, everywhere

Re: Checking Tach

#140 Post by photofly » Wed Dec 30, 2015 11:40 am

They don't read precisely the same because the prop speed is cycling slightly, and the measurement periods of the two units (over which the timing is averaged) are slightly out of sync. I did notice that they both showed the slight upward and downward trends together. The correlation is indicative that the periodic changes in measured rpm were a real phenomenon of the propellor and not a random artifact within the measuring system.

Without an alternative trusted measurement of rpm via a different mechanism I can't either confirm or deny that the full 20-50ppm crystal accuracy is achieved while looking at the prop. Perhaps a 'scope or FFT on the ignition pulses could be that source.

I can say however that the measurements "on the bench" of the powerline frequency and the close agreement between the two units "in the field" give me an extremely high degree of confidence in their power to measure the prop speed sufficiently accurately to confirm the prop is well within the allowed 4% tolerance - or not.

I can try an "interrupted laser beam" setup but that's going to take me while to arrange and it's only practical on the ground and not in flight.
---------- ADS -----------
  
“This isn’t flying, it’s falling. With style.”

NeverBlue
Rank 8
Rank 8
Posts: 907
Joined: Mon Feb 04, 2008 12:53 pm

Re: Checking Tach

#141 Post by NeverBlue » Wed Dec 30, 2015 3:09 pm

You need to have a reference...the ignition makes the most sense...then you'll know for sure.
---------- ADS -----------
  

NeverBlue
Rank 8
Rank 8
Posts: 907
Joined: Mon Feb 04, 2008 12:53 pm

Re: Checking Tach

#142 Post by NeverBlue » Wed Dec 30, 2015 9:19 pm

I can say however that the measurements "on the bench" of the powerline frequency and the close agreement between the two units "in the field" give me an extremely high degree of confidence in their power to measure the prop speed sufficiently accurately to confirm the prop is well within the allowed 4% tolerance 
Mearsuring a bulb is completely different than measuring a spinning prop through a windscreen with reflections from both and any other possible light in the area.

Completely different...

'kinda like testing snow tires for traction control in the mud...
---------- ADS -----------
  

NeverBlue
Rank 8
Rank 8
Posts: 907
Joined: Mon Feb 04, 2008 12:53 pm

Re: Checking Tach

#143 Post by NeverBlue » Thu Dec 31, 2015 9:42 am

The man with one watch knows the time. He with two is never sure. 
...unless the second one is Atomic...

Nice spin though!
---------- ADS -----------
  

User avatar
Troubleshot
Rank (9)
Rank (9)
Posts: 1291
Joined: Sat May 27, 2006 12:00 pm

Re: Checking Tach

#144 Post by Troubleshot » Fri Jan 01, 2016 11:27 am

//Personal attack removed by Sulako

It's 2016, a time of new beginnings and resolutions. Maybe yours should be to play nicer, and get fewer strikes. Just a suggestion.
---------- ADS -----------
  

cgzro
Rank (9)
Rank (9)
Posts: 1735
Joined: Wed Jul 25, 2007 7:45 am

Re: Checking Tach

#145 Post by cgzro » Fri Jan 01, 2016 11:52 am

reflections
I suspect that reflections are unlikely to be a problem given the speed of light and the distance covered would mean they are arriving within a few nanoseconds of the direct pulse and would therefore be integrated into that pulse as far as the rest of the circuitry is concerned. If however you were trying to make something for extremely high strobe rates this I suppose could be an issue (as it is for radio and optical fibre communications at high data rates). Photofly can no doubt describe how the analog and digital parts of the circuit react (or not) to such closely spaced pulses.
---------- ADS -----------
  

photofly
Top Poster
Top Poster
Posts: 6583
Joined: Tue Jan 18, 2011 4:47 pm
Location: Making aviation exhausting, everywhere

Re: Checking Tach

#146 Post by photofly » Fri Jan 01, 2016 7:59 pm

Really back of the envelope stuff: rise time on the input pin to the microcontroller is R-C limited to about 1.1 milliseconds, so it shouldn't be possible to trigger faster than about 900Hz (corresponding to 54,000rpm) - light travels about 180nm in that sort of time, so if you're worried about reflections from a mirror it would have to be that far away and you'd need a very bright light for it to be picked up by the sensor.

If I ever get me a decent waveform generator I can do some measurements.
---------- ADS -----------
  
“This isn’t flying, it’s falling. With style.”

NeverBlue
Rank 8
Rank 8
Posts: 907
Joined: Mon Feb 04, 2008 12:53 pm

Re: Checking Tach

#147 Post by NeverBlue » Sat Jan 02, 2016 9:49 am

Speed of light has nothing to do with it in the time frame in which we are measuring.

I will ask again...

If they work perfectly...and are soooo accurate...and never go out of cal...why the different readings?

None of you can answer that.

Suspect maybe....but prove...no

My methods leave nothing unknown...nothing.
---------- ADS -----------
  

photofly
Top Poster
Top Poster
Posts: 6583
Joined: Tue Jan 18, 2011 4:47 pm
Location: Making aviation exhausting, everywhere

Re: Checking Tach

#148 Post by photofly » Sat Jan 02, 2016 12:19 pm

NeverBlue wrote: If they work perfectly...and are soooo accurate...and never go out of cal...why the different readings?
Which different readings?
---------- ADS -----------
  
“This isn’t flying, it’s falling. With style.”

cgzro
Rank (9)
Rank (9)
Posts: 1735
Joined: Wed Jul 25, 2007 7:45 am

Re: Checking Tach

#149 Post by cgzro » Sat Jan 02, 2016 1:13 pm

Speed of light has nothing to do with it in the time frame
You cant have it both ways, either a) reflections are a problem, in which case the reflected flickering signal has to arrive after a sample period is complete and therefore add errouniously to the count and therefore RPM, or b) Speed of light is not relevant because its so fast the reflection cannot ever arrive outside of the sample interval given the small distances involved.

I suggested a) , photofly put numbers to it confirming a) and it seems you are now also agreeing with a) contrary to your previous assertion.
---------- ADS -----------
  

NeverBlue
Rank 8
Rank 8
Posts: 907
Joined: Mon Feb 04, 2008 12:53 pm

Re: Checking Tach

#150 Post by NeverBlue » Sat Jan 02, 2016 3:18 pm

:D
Please...I'll let you know when I agree...

You nor anyone else can tell me if any of what the photocell picks up are reflections or not.
---------- ADS -----------
  

Post Reply

Return to “Maintenance”