RPM sensor

[ShowMaster]

Since I'm flying more electric these days the optical sensor is a bit of a pain to mount. I'm thinking of substituting a Hall Effect sensor for the optical pickup and mounting it just behind the prop. A small spinner type plate with two magnets 180 deg apart turning with the prop should give the same rotational pulse the prop does when it breaks the sunlight hitting the photocell. I may not have to change the circuit to try this?
Anyone tried this yet or have a better idea to try?
SM

[herrkirk]

Hello!

I have the same idea. I fly a 450 heli and it's complicated to put an optical sensor in it. So I thought on placing 2 magnets (I already bought them!) on the main gear and attaching the hall sensor to the frame.

I removed the shrink tube from the FrSky rpm sensor and It consists basically on a optical sensor and a micro-controller.

Can I replace the optical sensor by the hall sensor? Can I connect the hall sensor directly to the hub?

[JakeStew]

I'm working on a CDI project at the moment, so I've done all this. Using a Hall sensor is very easy and by far the best way to do what you want.

You can check out the project thread here... (I know it's really long)
http://www.rcuniverse.com/forum/m_10662258/tm.htm

My website is here... (Project page)
http://www.electrofunnel.com/CDI-2012/index.htm

You can find the details of the hardware in the thread, or post questions there. You can contact me also, but I'm the software guy, not the hardware guy.

Basically, you'll need a couple resistors, a PIC 12F1840 (under $2), and the hall sensor ($1.00 or so). My code is pretty well commented so it should be pretty easy to see how I've done the timers and whatnot. I'm just now working on adding serial telemetry, so you can ask me if you need help or wait for my serial version to come out.

It will be easy for me to strip out the extra code so you can just use it for RPM telemetry if you're interested.


-Jake

[ShowMaster]

Thanks Jake.



ShowMaster
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[JakeStew]

In my other thread it was mentioned that you should be able to just directly connect the hall sensor to the sensor hub. It seems to be expecting two pulses per rev, so I imagine you'd just use two magnets.

[ShowMaster]

I'm thinking that an aluminum backplate spinner with two magnets mounted in drilled holes would work.
SM


ShowMaster
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[herrkirk]

I did it!

I connected a hall sensor (3144) directly to the FrSky hub. Glued 2 small strong magnets to the main gear of my heli, placed the sensor about 1mm away from the main gear and voilà!

The measurements are consistent with an optical tachometer.

[ShowMaster]

I did it!

I connected a hall sensor (3144) directly to the FrSky hub. Glued 2 small strong magnets to the main gear of my heli, placed the sensor about 1mm away from the main gear and voilà!

The measurements are consistent with an optical tachometer.

Super! Just to not wire it wrong is a schematic and part #/source posable? It should end up on wiki if your happy with you design and results.
SM


ShowMaster
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[herrkirk]

Sorry for taking so long...

I used this sensor: http://dx.com/p/hall-magnetic-sensor-mo ... -5v-135033. It uses an 3144 sensor probably from Allegro (although Allegro says it's discontinued). I could buy the sensor only but this one comes with a PCB which makes it easier to install. The board also comes with a LED that brights when the sensor is triggered, It's useful to adjust the distance between sensor and magnet.
I looked at the datasheet and realized the board inverts the + and - pins, so watch out! Then I replaced the connector with the wires that came with the optical RPM sensor and connected to the hub.

Hall sensor attached to the frame using a strong double sided tape and a small but strong magnet glued to the main gear using CA (I'm using 2 magnets 180° apart)

Here's the sensor hub just to show how was the RPM sensor connected.

[MikeB]

r131 of ersky9x allows a blade count for 1 to 50 for the RPM sensor.

Mike.

[michel49]

I would be very happy to have a brushless RPM sensor, in the same way as EagleTree...

[jbeebo]

michel49, i was thinking something similar. Why not pick up electrical pulses from one of the motor phases? Would eliminate magnets. Current sensor would probably work best.

Might need a divide-by to calculate rotor speed based on motor magnetic pole arrangement and gear ratio, but sounds like MikeB already has capability in firmware...

[michel49]

@ jbeebo

Of course...but I'm not able to interface with the FrSky hub; I assume it is a I2C one. anybody to design it ?

[Kilrah]

FrSky hub's RPM sensor input is just X pulses per rev.

[Thorvald]

Exactly, nothing special about the RPM sensors. I've done tests with all kinds of makes and models (usually ones for Nitro Helis). Work just fine. Currently I'm using a Spektrum crankcase sensor and a Multigov gear sensor (both of them just x pulses per rev).

Just remember you are currently going to get the RPM of engine/motor, not the head speed. For Taranis we may be able to resolve this via the new Lua support in 2.0 but for the other radios which don't support LUA you would need to add a gear ratio option to the RPM calculation source code if you want real time head speed (I have sample code on Github in my repository that does just this in 2940 firmware but I'm holding off on 2.0 to see what transpires with Lua).

Also if you are going to put magnets on your gears make sure you put a magnet (upside down) on the opposite site to balance the gear.

However for electric, I'd just use any off-the-shelf RPM-via-ESC sensor, Eagletree for example (Spektrum is just a rebrand of the Eagletree one).