RPM sensor for model aircraft

[MusMus]

Hi guys. long time watcher of the tube, first post.

Im attempting to equip my model helicopter with a (commercial, Hobbywing branded) RPM sensor on the three phase motor to get a governor function for stabile RPM under varying loads.  it works fine, but get interference from a digital dataline on the receiver.   

goal is to make the cirquit more resistant to the interference. i tried to shield the cirquit with a grounded copper wrap, but within 5cm the reciever still gives interference.   i dont have the space on the model to route the wires with enough clearance and i need a reliable solution.

I have dissected the cirquit. not sure about the two tiny black smd components near the input.  it measures 1k8 but might be an inductor.
the cap in series has been desoldered and measured 3,3nf.
Can someone help me interpret the cirquit workings?  i dont see the reason why they would measure 2 motor phases.  perhaps to make it not need a common ground?  or make it work between 8 to 60 volt motors.

The cap at the output just forces a clean blockwave.
 The 8uF at the input seems way to high to act as a low pass filter, as the motor coil switching frequency is much higher at around 3kHz at max motor rpm

The diode at the far right i added myself to make it drain-only for the pull-up resistor in the flight controller that does not like voltages above 3V3.  Current RPM sensor VCC is +5V.


First scope image is the uninterrupted output blockwave at 1/2 motor rpm. next the the jammed up one, that confuses my flight computer.
Next is one motorphase and ground measured, and then across two of the three motor phases.
The ESC on these motors work with PWM on 8,12 or 16kHz to regulate the motor torque. RPM is dependend on load and battery voltage, the ESC just follows the magnet timing.  Some ESC's have a governor option build in, but the flight controller can do feed-forward and maintain RPM much better.

Last two pics are the digital signal pulses measured on the receiver.   its the Futaba S-bus protocol from receiver towards the flight controller for control of model itself.   The interferens seems to be radiating mostly from the receiver PCB, but dataline and supply lines toward the rest of my electronics are suspect.

Any help would be highly appriciated.  Soldering up my own bandpass filter is also an option, passive or active.  I orderd a few bags of capacitor and inductor assortiments on Ebay to tinkle with.  lacking experience wih RCL ciruits and a decent scope might make things difficult.....


edit: added photo's

UPDATE: i cant find any SMD inductor components with a high DC resistance, so the black unmarked compnent must be a simple 1800 Ohm resistor.
The inductor in the schematic should be replaced by that.
So this input cirquit is not an LC-oscillator but a RC low-pass filter. The resistor in parallel with the 3,3nF might just be there to drop the input voltage down to workable levels for the opamp.  Some motos run @ 60V
That makes a resistor voltage divider 47K and 1K8.   Anyone know how this setup would influence the cutoff frequency of the low-pass fliter?

[firehopper]

I've seen rpm's measured by 3 methods, 1 your using a direct wire connect, 2 via a optical sensor( prolly not practical for you) and 3 via a tiny magnet and sensor.

[KC0PPH]

Why not buy a genuine Futaba part?

I have used their Optical ones with great success (Airplanes not Helis)

I believe their optical ones can be used on a heli facing up

[MusMus]

interesting, diddn't know that existed. 
I have not seen anyone on the helicopter forums even try to use an optical tach sice i started in this hobby 10yrs ago.. Its all simply Hall-effect +magnet for combustion engines and phase sensor for electrical. 
 
Some problems i see with the optical tach. That one outputs S-bus digital data.  the flight controller expects a simple blockpulse.  Cost is a factor:  if it works i want at least 5 of my fleet equipped, thats $250 instead of the $25 i spend in components up front now.

Mounting is an issue, space is tight and not all angles avalible.  Vibrations on these craft are abysmall when flying hard....or simply terribly when flown slowly.  any loose bolt, you are guaranteed to drop in flight.   The less physical hardware on it, the better, especially when rotating.


This problem seems easily fixable with a little more knowledge about RCL osccilating cirquits or bandpass filters. Just cant wrap my mind round the 47K resistor in series with the paralleled L+C and what that does in this ciruit.

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i found a bit of info in the datasheet of the LM393 dual-opamp.   the second opamp is unused and inputs simply bound to grount and output floating. In the datasheet it is advised to tie each input with a resistor to ground and/or vcc.  a handfull good ways to do it are presented.   perhhaps the RF signal is simply disturbing the unused half of the chip....    need to desolder those leads and find out i guess.