Mounting a rare earth magnet in a thin plate

Karl Townsend fired this volley in news: snipped-for-privacy@4ax.com:

A drilled hole works fine for a chopper. You can also balance your wheel/spinner backplate easily using drilled holes.

I personally think the whole idea of direct-sensing of the motor shaft on a model airplane is a fool's errand.

Why not just use a piezo-electric vibration pickup anywhere _near_ the engine, and be done with it?

LLoyd

The fastest I could make a reed relay cycle was around 2 KHz, by overdriving it. I've operated laser diode switches above 3 GHz. jsw

Will it fall back to open-loop if the RPM sensor fails or goes out of range? I'd be much more concerned with the magnet or counterweight flying out and the aircraft self-destructing.

You could mount the magnet stationary on one side of the shaft and couple it to the sensor on the other side with a symmetrical steel bar on the shaft.

Might be a good application for J B Weld.

+1

Adding mass to a rotating crank can be iffy, adding considerable vibration and shorten crank life. (Ask Ford with its cut-down-V-8 V-6 diesel. A CA fleet truck dealer told me in 2007 that only two of the hundreds they sold had NOT come back for major engine repairs within a year. I ended up with the Tundra.) Balancing with an equal weight of steel rod 180-degrees out would double that mass and any problem associated with it.

LJ's 2-cents: Optical seems the best route. If it's a dirty enviro, shield it, wot?

Larry Jaques fired this volley in news: snipped-for-privacy@4ax.com:

Still... why not a vibrational pickup? Then there's _nothing_ to get dirty, no imbalancing of the engine, and no 'attachments'. Just mount the pickup _near_ the motor mounts and be done with it.

They've got them in teesy sizes that will handle up to a couple of hundred KHz, can consume NO power (some active ones will pull a couple of mA.)

Lloyd

Maybe some of the nice retro reflective tape that 3M makes? George H.

Hi Tim, I've never done light detection in full sunlight, so I may be full of shit. But there is some nice 3M retro reflective tape that Phil H. (of SED) turned me on to. As long as you have enough dynamic range in the detector (to take care of full sun) cna't you you then AC couple the signal? (White leds are blinding these days.) (Will it kill you if you get a few false counts as full sun blinks through the spinner onto the detector. That's going to be a very small solid angle) I've no answer for dirt and grease.

George H.

I am starting to think in terms of opto -- but usually the decision tree for industrial equipment for optical vs. magnetic starts with "will there be dirt on the surfaces?" -- if the answer is "yes", then the following box says "use magnetic sensing".

The spinner comes with big lightening holes, much larger than the magnet I'm thinking of using. I'm considering making up some high strength epoxy putty with some chopped carbon fiber strand and microballoons, and holding the magnet in with that. It's kind of a cro-magnon solution, but it may work.

Now, if only someone made rare-earth magnets in the form of little screws, I'd be all set!

Ah, tnx for that. Heard of "Hall effect", didn't know that commonplace articles implemented it. Where would I look in old junk to find one?

Someone gave me a DecWriter II in 1990, when it was at archaeological-artifact stage of life, which I played with for a while. The opto-mechanical unit for spacing eventually failed and I was never able to get it working again. I suppose the similar little wheels in a mouse would have the same potential for refractory NFG-ness, which is why I thought of but didn't try that for my (real) mouse tach.

Crank position sensors (they'll have a hall sensor and a magnet).

Laptops and flip-phones often use them to sense that the thing is closed.

Be warned that the laptop and flip-phone ones achieve very low power consumption by only turning on about ten times a second -- at all other times they pretty much ignore the world. Also be warned that this is the biggest market share by far -- finding a Hall sensor that'll work on a

12000 RPM engine takes some digging (I've got part numbers, if you want to order some from DigiKey).

Yes. It's called a hall efect sensor

An old Omni distributor generally had 2 of them. (Aries and reliant too)

That is not necessarally true when it comes to model airplanes. I once saw a "speed job" a small controll line model designed to go fast, that used a single bladed prop, with a conterweight with an engine speed in excess of 10,000 rpm.

A sort of left hand turn off the topic but why not just use an optical tach for setting static speeds and (perhaps) and acustic tach to calculate the increase in rpm when flying?

One example:

Vibe tach? Nevahoiduvit. Are they available?

John B. fired this volley in news: snipped-for-privacy@4ax.com:

What kind of chugging hunk of a model airplane engine goes 10K? (except at a high idle?) There are lots of them out there that consistently tach

29Krpm.

Lloyd

Larry Jaques fired this volley in news: snipped-for-privacy@4ax.com:

All day long. They're almost a commodity item. Anything with a reliably repeatable cyclic vibration is a candidate for the method.

They even sell sub-$20 elapsed-time meters for gasoline engines that are only activated when vibrated by the running motor.

Lloyd

"Lloyd E. Sponenburgh" fired this volley in news:XnsA321DF3FB2774lloydspmindspringcom@216.168.3.70:

um... and some that do 40K.

Lloyd