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Thread OP
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Discussion
IC engine and FrSky smartport RPM sensor... A coil works! A capacitor works better!
Hi everybody,
This can be a quick question or a long discussion, I don't know yet.... I have a FrSky Smartport RPM sensor (the one with 2 temp sensor connections as well) and it is designed to be connected to the motor-wires of a brushless motor. According to the specs, the sensor should work from 2S to 6S, so the imput should be able to handle from appr 6,6 up to 25 Volt signal. However, I don't run Elelctrics, but a gasoline engine, RcExl ignition. I had slight hope that connecting the RPM sensor to the tacho output would do something, but (duh!) it didn't, which is quite logical in hindsight, as the tacho output only has about 5V signal.... it is also a blockwave that does not change polarity like the signal from the motorwires would. Does anybody have a quick and easy means to get an RPM readout from that Ignition? Or would it be possible to place, for example a coil close to the timing magnet as a sort of pulsegenerator? I have some basic understanding of what electronics do, but zero actual experience or detailed knowledge. I am able to understand explanations (I am an engineer after all) but lack the skills to figure it out for myself, electronics simply is not my forté. I would be thankfull if somebody could put me on the right track.... |
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Last edited by Brutus1967; Feb 24, 2019 at 10:38 PM.
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Thread OP
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Well, since nobody answered, I just decided to mess around a bit for myself with a few coils I had laying around....
Beginners luck maybe, but first try out, it worked. Since these coils were unmarked I have no idea what value they are (and no way of determining for now, but I can ask a friend with a scope and a function generator if he knows a way). I noticed that especially at idle, the coil has to be pretty close to the hub to pick up a wave, but somehow the FrSky electronics need a treshold value: once it starts detecting, it keeps detecting. I confirmed the RPM measurement with a separate tach as absolutely correct (both optical tach and the Taranis showed the exact same RPM). Just a male JST connector, some cable, and a coil. This cable happens to be semi coaxial, don't know if that is needed or not, it was what I had on hand, and it being the signal cable of an old flight sim interface, I figured it would definitely be suitable. (bad) video:
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Last edited by Brutus1967; May 20, 2017 at 03:30 AM.
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Thread OP
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Coil, solenoid, some piece of weak-iron with a wire wrapped around it....
My english is pretty good, but I have not all proper technical names for electronic components readily availlable in the back of my head If you look a bit closer, you'll see there is an opposite connector to the one standard in use with the FrSky RPM/temp. board... Yes, an inductor, directly plugged into the two sensing wires of the brushless RPM sensor.... |
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Quote:
So yeah. The coil is wrapped around a ferrite core and is called a choke or inductor. |
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Thread OP
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Ah, OK... I was not aware of the different terms: both, choke, inductor, coil and solenoid translate in my language to one single word "spoel" (which would also translate back to the english "spool"....
I guess I have to play a bit with the value of the choke, as at low RPM (coincidentally just around idle) it starts to drop pulses and read wrong. Above 2500 RPM and up to full (10400 in this case) the reading is within about 2% of my other tach.... |
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Last edited by Brutus1967; May 20, 2017 at 02:54 PM.
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Thread OP
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Nope... that was the first thing I tried (see also my first post).
Hang on a minute, I'll try and get a pic.... |
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Last edited by Brutus1967; May 20, 2017 at 03:11 PM.
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Thread OP
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According to the literature, there is nothing that says a solenoid MUST have a moveable core, or has to be used to actuate something...
https://en.wikipedia.org/wiki/Solenoid The word solenoid itself strictly refers to a helical/cylindrical winding of conductor around a straight core (actually it strictly means cylindrical shape or tube like shape). Although admittedly very unusual, and for clarity reasons undesirable, the word solenoid is correct even for the device in the pic in my previous post |
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You are correct. A coil wound in that manner is referred to as a "solenoid" coil.
No where is a movable plunger a requirement. |
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Thread OP
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Temporary obsolete post
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Last edited by Brutus1967; May 30, 2017 at 01:41 PM.
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Not anything spectacular, just something fun:
Made a logical switch that opens the throttle a bit when RPM gets below specified idle. It is not a real stable RPM control, but amongst others it prevents a cold engine from stalling, and it automatically opens the throttle a bit before starting, to come back to idle after a few seconds, without having to fiddle with the throttle stick... Really practical? No idea yet... for now it is more intended as a fun way to learn working that Taranis programming... If you watch closely, you see the throttle action, the Taranis does that all by itself...
EDIT: above vid was with still a very simple single logical switch (RPM below treshold, throttle opens slowly, RPM above, throttle closes slowly). This results in a control that continuously and slowly oscillates around the setpoint. Updated it with an array of 10 switches with each a different treshold value, and 1/10th of the influence. The outputs of those ten switches are added and the result is something that more resembles a kind of PI controller: at some point one half of the switches cancles the other half out, and the system kinda sorta finds an equilibrium. It is not a PI controller, but it does a fair job imitating one. The number in the top of the display represents the throttle servo position (and thus the control response)
Apologies for the 90 deg tilted image... the Original file shows normal, I have no idea what happened here |
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Last edited by Brutus1967; May 30, 2017 at 03:48 PM.
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I have been searching for an RPM sensor like this, I can't believe FrSky has no solution available in the market for gas users.
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