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RPM sensor for MGL and other Tacho systems

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RPM sensor for MGL and other Tacho systems

Postby diablo » Mon Jul 06, 2015 12:23 pm

There are a variety of ways that can be used to obtain engine RPM data, some are more reliable that others. I have never liked the Jabiru standard setup as it is quite fragile and prone to errors. It needs to be set up accurately with a very small gap (10 thou).

It is also possible to monitor RPM using the output of stator winding. However, the output of stator, has to be attenuated using a resistor and due to noise. I have tried this and found it to be unreliable. It often gives mixed results.

A few owners have experimented with hall effect sensors and it seems to be the most reliable method of monitoring engine RPM. Hall effect sensors detect changes in magnetic field. As there are two large magnets on flywheel, each time one passes past the sensor, it produces a signal. Hence each revolution produced two N pulses and four S pulses. If the MGL or other tacho instrument is set to count the number of pulses and divide it by two or four (depending on what sensor you have), it will display engine RPM.

I obtained a hall effect inductance proximity sensor from ebay and set it up. It seems to work very well on a test bench. I intend to set it up on my aircraft with the MGL efis. I have made a bracket that fits where the standard tacho post is. There is no reason why it cannot be used with the standard Jabiru Tacho. As long as tachi is configured corrctly it will work reliably.

IMG_4316.JPG
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IMG_4315.JPG
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The recommended sensor for this applications si the following:

http://www.ebay.co.uk/itm/311320797500? ... EBIDX%3AIT

Specifications are :

Type: NJK - 5002C
Appearance: M12mm cylinder
Output: NPN three wire normally open type
The detection distance: 10 mm
Supply voltage: 5-24VDC,6-36VDC
Output current: 200 mA
Objects can be detected: permanent magnets
Switching frequency: 320 KHZ
The shell material: copper
Wiring: brown-Power+,blue-Power-,black-signal
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Re: RPM sensor for MGL and other Tacho systems

Postby diablo » Sat Aug 22, 2015 6:39 pm

here is how I fitted the sensor to my set up:

The sensor operates with a power input voltage of anything between 5V and 30V DC. Distance from sensor to magnets can be anything up to 10mm (mine is set to 5mm). This is great as 10 thou is quite a small gap and very easy for things to go wrong with such small gaps between fixed and rotating masses.

The setup requires a bracket that should be made of 1/4" aluminium with a 12mm hole for sensor and holes for fixing to spider. Can be fixed using 1/4" holes in centre of spider. The sensor needs a +12V (brown WIre) and an earth (blue wire). The black wire is the signal output wire and should be connected to the EMS, EFIS of VDU.

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IMG_0783[1].JPG
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The sensor detects the South Pole of the exciter magnets (four poles) and produces a pulse each time the south pole flashes past it (4 times per complete revolution). Therefore your EMS, VDO or EFIS device must be set to count one revolution every four pulses.
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Re: RPM sensor for MGL and other Tacho systems

Postby mbentley » Thu Apr 07, 2016 2:55 am

How is the tach sensor working? I'm thinking about using one like that with my ISS tachometer. http://www.aircraftspruce.com/catalog/inpages/issgauges9.php
Mike Bentley
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Re: RPM sensor for MGL and other Tacho systems

Postby diablo » Thu Apr 07, 2016 11:31 am

I have it installed on my aircraft and to date (about a year) no probs to report. Here is a pic with tacho sensor powered by a 12 volt PSU, used to RPM on a lathe chuck and then connected to a Tiny Tacho to monitor RPM.

0 RPM
IMG_5077.JPG
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2250 RPM
IMG_5078.JPG
IMG_5078.JPG (367.64 KiB) Viewed 6516 times


3000 RPM
IMG_5079.JPG
IMG_5079.JPG (380.65 KiB) Viewed 6516 times


The tiny tach is steady but seems to go up in 30 rpm chuncks, hence slight difference between RPM on lather and tacho. But, it seems to work reliably throughout the Jabsiru's rev range ...
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Re: RPM sensor for MGL and other Tacho systems

Postby mbentley » Fri Apr 08, 2016 12:54 pm

Thanks for the reply, here and on the Jab/Camit forum. I've got one on order.
Mike Bentley
Kitfox Model IV-1200
Jabiru 2200A #438
Rotec Aerosport LCH Heads
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Re: RPM sensor for MGL and other Tacho systems

Postby FoesFlyDennis » Wed Feb 08, 2017 8:57 pm

Thank you, diablo for sharing your solution to a frustrating problem.
All I wanted to do was replace the integrated under-performing Jabiru alternator with the Rotec 45 amp unit. Then I realized my Dynon Skyview was getting it's tach input from the old alternator-- Woops! I'll skip the details of how much time I burned searching online for how to make the tiny Jabiru induction sensor work with my SkyView... The solution you suggest makes perfect sense and I have mine on order.
At least two of the photos you originally posted here have been dropped. If not very inconvenient, could you repost them? Since you state that you attached your fabricated bracket to the 'spider', I'm curious as to its orientation. I assume my new tach sensor needs to read from the same face of the magnets as the ignition coils. Since your bracket is a simple L shape, I don't see how that orientation can be accomplished. Makes me wonder if the magnets can be read reliably from the 'aft side'. If too much trouble, no worries. Am sure I'll figure it out. Thanks again!
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Re: RPM sensor for MGL and other Tacho systems

Postby diablo » Fri Feb 10, 2017 3:15 pm

Hello foesflydennis

happy to be of assistance. I have made a number of sensors that will fit on existing tacho post. Here is an image:

IMG_0052.JPG
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I am selling them at cost (a little more than china sensors) at £12.00 which will go to maintaining the jabiruownersgroup site running. Please let me know if you want one. It is just as good but not as elegant. However, it has the added advantage of fitting on the existing tacho post and gap can be far bigger at 5-8 mm from magnet.

The LED on top will light upwhen it detects the magnet (2 x pulses per revolution). Red wire goes to battery and white wire is signal ( a healthy 5V).
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Re: RPM sensor for MGL and other Tacho systems

Postby FoesFlyDennis » Fri Feb 10, 2017 7:31 pm

Now the pictures are all there! Very helpful too. I had wondered if the magnets could be read from that side face, rather from outside the flywheel radius. I ordered the same part number Hall-effect switch you mentioned above from Amazon and it is arriving today. Will be so happy to get this significant instrumentation issue resolved! Thank you much!
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Re: RPM sensor for MGL and other Tacho systems

Postby diablo » Fri Feb 10, 2017 8:15 pm

Hard disk on server needed an oil change to correctly interweb ... seems to be running a lot smoother now, also changed the head as the magnetronic interferometer was not arching as it should have been (a bit like a spark plug) and adjusted the I/O gap on the RJ45 connector as we were losing data compression (likely to be down to a gap being too small).

All hunky dory now. Running a bit lean but hopefully we will get more members posting which should sort that out ...
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