Mounting a rare earth magnet in a thin plate

Problem restatement: get a rotation indication using a Hall sensor. So, put the magnet on the fuselage, the Hall sensor between the magnet and the backplate, and drill/tap the spinning part for a small ferrous screw. When the screw nears the field, it will cause a spike in the magnetic field (as it becomes magnetized). Works with a seek coil, too.

Loctite will hold the threaded part.

Reply to
whit3rd
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I was being conservative as I saw the plane in, probably the mid

1950's, and I really have no recollection of actual RPM as we just tuned them by ear and flew them :-)
Reply to
John B.

Interesting! More research is in the offing.

Yeah, $13.98 for a vibration hour meter for ATV/cycle/etc. on eBay.

Reply to
Larry Jaques

Hour meter and tach are totally different tech togh. The hour meter just counts time when it is buzzing. No speed info involved.

Reply to
clare

Much simpler to use variable reactance sensor - fine coil around a magnet, makes signal when ferous screw passes by.

Reply to
clare

Male that variable "reluctance"

Reply to
clare

snipped-for-privacy@snyder.on.ca fired this volley in news: snipped-for-privacy@4ax.com:

Clare, Clare... (sigh) One can make a tach from a transducer as simple as a 75-cent electret microphone. Some AGC, peak detection, and a little band-pass filtering, then simply square up the signal and count it.

And all those features are available on a single analog chip. But done in DSP, it can be done on a $4.00 Cortex.

Think about it. "Technology"? Yeah, 1970s technology.

Lloyd

Reply to
Lloyd E. Sponenburgh

Right, but you see that I replied under the reference to hour meters vs. the reference to cyclic vibration. I only LOOK dumb, clare. ;)

Reply to
Larry Jaques

Step away from the bar, sir. Spelling's a ferris wheel ride, wot?

Reply to
Larry Jaques

Yes, both can be done with very simple technology - but the hour meter can be even simpler. Not meaning any less expensive, however. Just a missed pulse detector - can be as simple as a 555 with a trip and reset. As long as there is a signal to reset before the timing period expires, it keeps the clock running. (that's mid-seventies tech)

Reply to
clare

It was more than spelling though - both reactance and reluctance are real and spelled properly -----

Reply to
clare

snipped-for-privacy@snyder.on.ca fired this volley in news: snipped-for-privacy@4ax.com:

Are you someone who enjoys deliberately diverting a subject just to ruin a conversation, or are you so thick you don't know you're off-track?

WE were talking about implementing a tach. The reason I mentioned ET meters was that they often use the same acoustic transducers as vibration tachs to trigger them. And you know that's why it came up.

You're getting as bad as that 'CNC expert' asshole who's always trying to wreck other people's threads. Get back on the subject - if you know anything about it.

Lloyd

Reply to
Lloyd E. Sponenburgh

Reference: ferous. 'Nuff said?

Reply to
Larry Jaques

Nope. I believe it was YOU who brought up the hour meter

And I just pointed out they are not necessarily the same - and to get an accurate ET meter is a different and less complex challenge than getting an accurate tach reading.

I've run a few "audio" tachometer applications on PC and smartphone.Dead accurate on one engine, and way off on another virtually identical engine (with different exhaust), and not even close on another engine. If sensing (non audio) vibrations the same thing can happen - a different engine mount stiffness, and the tach is way off. Getting them tuned can be a challenge

I used to use a piezo tach and timing light on deisel engines a couple decades ago - which did work very well.(clamped the sensor onto an injection line - read the pressure pulse as the line expanded ever so lttle under the several thousand PSI injection pulse)

I've had the same problem with photo-tachs, where for some reason the tach would cycle from dead on, to 2x, 3x and even 4x speed when reading off the prop at certain speeds - don't know if it had to do with a resonance in the drive, or what. Again, this was a few years back, and the capability of "simple" electronic circuits has increased with leaps and bounds in the intervening years - but there are also a lot more "cheap" solutions out there that are just plane poorly engineered and implemented - always someone willing to make something cheaper than it should be.

Reply to
clare

snipped-for-privacy@snyder.on.ca fired this volley in news: snipped-for-privacy@4ax.com:

Pffft! We're talking about a one-lung model airplane engine with a straight pipe! Even if it were a twin or a radial, it would represent the same sort of vibration profile, which is almost always power-stroke induced. Just more of them per rev.

I've used a lot of vibration tachs, too, and never had one misrepresent the correct RPM on a single-cylinder engine.

Lloyd

Reply to
Lloyd E. Sponenburgh

"model aircraft" engines run the gamut from 2 stroke compression ignition mosquitos to 4 stroke spark ignition singles and twins, to 3 and 5 cyl rotary and radial 4 strokes, and wankels.

Our local RC club flies them all. Twin engines too (and even a 4 engine monster) Vib tachs don't work very well to sync twin engines.

And they go nuts when an (2 stroke) engine starts "4 stroking". If a spark ignition 2 stroke doesn't 4 stroke coming off load, it's running too lean.. Don't know (or pretend to know) about a glow engine but I know my old enya 4 stroked unloaded when it was running properly.(WAY too many years ago)

Reply to
clare

snipped-for-privacy@snyder.on.ca fired this volley in news: snipped-for-privacy@4ax.com:

And you also know that anyone interested in tach-ing a model isn't interested in performance outside the flight power envelope.

And no, I doubt they'd be very interested in the RPMs running all the 'exotics' like Wankels, either. (got a few thousands of hours RC'ing myself).

I'm betting his is a performance model like a pylon racer or a ducted-fan job. There's not much other cause to need to know the speed of the engine. Just for the record, the high-performance engines NEVER 4-stroke unless they're broken. They pretty-much run full-load and full RPM all the time except when coasting. And most of that class are single- cylinder. (mass has its costs)

You'll bring up ANY minor point you can to prove your own personal argument, won't you? It seems you'll do that even if it doesn't match the general subject or the specifics of what's being discussed.

Engineers work on the problems at hand, Clare, and they only bring up spurious side-bar crap when they think it might affect the actual goal. You never ASKED what sort of engine, and had absolutely no insight into why someone might want to know the RPMs of an RC engine (right there, tells me that you're only spouting crap from a catalog, and not from actual RC-ing)

B.S. salesmen are the ones who raise all the issues not related to the project. 'Guess you were a technology salesman.

Lloyd

Reply to
Lloyd E. Sponenburgh

Any 8 cylinder automotive electronic distributor. 8 cylinders firing once every 2 revolutions at 6,000 rpm equals 8 times 6,000 divided by 2.

24,000 trigger events per minute. should work. Just adapt the mechanicals, reduce, whatever, but the electronics in a distributor are VERY robust. >
Reply to
Steve Walker

[ ... ]

Hmm ... near where moving magnets are sensed -- if you can find "old junk" which is new enough. :-)

The problem with the DecWriter-II is that the chopper was a thin metal disc stuck to a hub with double-sided tape. When the temperature gets too hot, it can shift off-center and stop being sensed properly. (The head will bang into the stops at one end of travel or the other.) (Ask Me How I Know This. :-)

So -- the problem there was not with the robustness of optical sensing, but with the robustness of the mounting of the chopper wheel. A very different matter. If you drill a number of holes through the spinner, and put the sensor inside (assuming that you want this to be sensing during flight, instead of just on a test bench) it should be well enough protected, and the spinner will certainly stay put. It is not mounted by glue, I hope. :-)

Enjoy, DoN.

Reply to
DoN. Nichols

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