RPM Sensor

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jhsa
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RPM Sensor

Post by jhsa »

Hi, me and my son, we were just playing with a little toy helicopter and I just had a crazy idea..
I know there are rpm sensors for the frsky receivers and all that.. but what about an RPM sensor built in the radio that would show the rpm on the telemetry screen.. just point the radio at the prop as you have to hold it anyway..

Any ideas?

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Kilrah
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Re: RPM Sensor

Post by Kilrah »

The Futaba 9Z had one :)

I've never used an RPM sensor again since I stopped flying fuel-powered models :D
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Re: RPM Sensor

Post by Crashj007 »

When you say "radio" do you mean "transmitter held in your hands" ??
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jhsa
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Re: RPM Sensor

Post by jhsa »

so, it would be a good idea then..
Only one thing, I have no idea how to do it.. can do some research about the electronics but this would need messing with the code. :?
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MikeB
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Re: RPM Sensor

Post by MikeB »

If you want something like that, I reckon it would be a function to put in the TelemetreZ board.

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jhsa
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Re: RPM Sensor

Post by jhsa »

ageed, but I was thinking more like building my own.. I like to build electronic things.. I had a lot of fun with the sound module's board.. ;) :D
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RPM Sensor

Post by ShowMaster »

For brushless electric I saw at a hobby show a sensor that you connect across any two motor wires only. It had a digital display no telemetry and was $10 at the show. I now wish I had bought it just for the circuit.
If this idea works it's what we need to replace the optical sensor.
So what kind of a pulse train does a brushless motor have across it's wires? I've never put a scope on it.
Just food for thought.
SM


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jhsa
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Re: RPM Sensor

Post by jhsa »

I was tinking more of an optical.. to point it at the prop or rotor.. like this we wouln't be limited only to electric motors.. I have also Internal combustion planes.. and it's nice to tune the motors..
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Re: RPM Sensor

Post by gohsthb »

http://homepages.paradise.net.nz/bhabbott/tacho.html
There is a good schematic here. I used a similar circuit, it is written down at home somewhere. If I can find it I will share it. I have not built/tested the circuit I linked to. What you are looking for is a light dependent resistor(LDR) and comparator. As the light level changes on the LDR, blades passing, the comparator picks that up and amplifies the signal for a uC. The uC then simply needs to time how long between blades to get the rpm.

-Gohst
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jhsa
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Re: RPM Sensor

Post by jhsa »

I was thinking about a photo transistor.. but I guess normally an LDR is used
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Re: RPM Sensor

Post by Clivew »

Ahh, last time I used a fhoto transistor was over porty years ago, when I made a tacho
for my fylon racers. Can't seem to pind the circuit now :lol:
(ferhaps we should use enigma machines here por more security ;))
Last edited by Clivew on Tue Jul 10, 2012 10:46 pm, edited 2 times in total.
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jhsa
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Re: RPM Sensor

Post by jhsa »

are you making pun of the foor Fortuguese?? :mrgreen:
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Re: RPM Sensor

Post by jhsa »

gohsthb wrote:http://homepages.paradise.net.nz/bhabbott/tacho.html
There is a good schematic here. I used a similar circuit, it is written down at home somewhere. If I can find it I will share it. I have not built/tested the circuit I linked to. What you are looking for is a light dependent resistor(LDR) and comparator. As the light level changes on the LDR, blades passing, the comparator picks that up and amplifies the signal for a uC. The uC then simply needs to time how long between blades to get the rpm.

-Gohst
I do like this one gohsthb.. now how could we interface something like this with er9x?? would it be possible to use the same serial connection as the telemetry?
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Re: RPM Sensor

Post by MikeB »

Some time ago I made myself a wattmeter, but included an optical RPM sensor on a long lead, worked well. If I get a moment I'll look out the circuit. It used an optical sensor that I think was almost a minature solar cell. It seems to work well, even in artificial light.
The wattmeter also included a single channel PPM output with a pot controlling it so I can run and measure a motor setup without needing the radio hooked up. It even measured a PPM input from the radio, and logged all the reading to a serial port.

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Re: RPM Sensor

Post by jhsa »

thaks for that Mike.. But did you hook it up with er9x? or was a completely separate project?
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Re: RPM Sensor

Post by MikeB »

Completely separate, I did it before I found er9x.

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Re: RPM Sensor

Post by jhsa »

would it be possible to connect it serial to the er9x and take advantage of the frsky rpm feature? then could be installed inside the radio with the sensor pointing out
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Re: RPM Sensor

Post by Clivew »

Don't make Mike's brain exflode, we need him!
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Re: RPM Sensor

Post by jhsa »

I bet you also don't know what plaps are..
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Re: RPM Sensor

Post by Clivew »

plaps?
You mean 8"9l? Or didn't you get today's code? :lol:
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Re: RPM Sensor

Post by Crashj007 »

JHSA, ITYM, "plafs?"
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Re: RPM Sensor

Post by jbeebo »

@Showmaster,
Figure1 of this article from Digikey shows motor control timing chart for 3phase BLDC motor. They show a sensor motor design, but sensorless (typical RC motor) operates the same way. Only correction I would make to Figure1 is usually only one FET is PWM (either hi-side of low-side it doesn't matter) while the other one is "on". Regardless, you can pick up 1 motor phase and read the current or voltage to determine motor speed. I would optically isolate the motor signal from the telemetry. From there, calculate rotor head speed or prop speed based on motor poles and gear ratios.

The stuff I work on uses 3phase BLDC motors with hall sensors. We do closed loop position control and velocity calculations based on Hall feedback, but motor feedback can easily be substituted.

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