About pots vs Hall Effect sensors

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Q. Why don't ProModeler servos use Hall Effect sensors instead of pots (potentiometers), aren't they better?

A. This is something we're asked about a fair bit via email and even by phone. The short answer is . . . in theory an HE sensor (Hall Effect) should be better than a pot because it has a nearly indefinite lifespan. So because it lasts longer than pots it should be a no brainer, right? No, not really because in practice it isn't such a clear cut decision. The ultimate reason we remain leery of using Hall Effect sensors is because of their failure mode. Let me explain.


It's all about how:

Basically, when a pot gets to be a problem it lets you know with increasingly poor centering. Or maybe with a bit of a jitter at some point in the travel, or neutral (not to be confused with feedback oscillation). But typically this takes a while to happen. Especially with our servos because we use premium Nobel million-cycle pots (the best in the business and the genuine Japanese item instead of the far east knock-off).

The point being, you usually get plenty of warning a pot is going south. However, with a Hall Effect sensor, the failure is usually poof (like a light bulb). This is because it's an electronic-component that dies suddenly vs. a mechanical-component that fails gradually. You see, everything dies eventually but it's in the 'how' something dies where good engineers earn their pay. How is an important engineering consideration.

So on the one hand you experience an aileron or elevator with a 'nervous' spot, which is basically telling you the servo needs service. On the other hand, your model is flying along when it craps out, suddenly! Honestly, ask yourself, which type of failure would 'you' rather experience? You see, it's not a matter of quality, but more like only God is perfect. This is why shit happens became a saying in the first place!


It's about cost:

So why do some companies use Hall effect sensors. Well, for one, Hall Effect sensors are a 'lot' cheaper than top quality mechanical potentiometers. Note; we could also go cheaper but instead, we're using the top brand item versus the clone product. And as an aside, the knock offs are claimed to also have a million-cycle life but in 'our' testing this isn't always true (some do, some don't). Quite honestly, cheaper is 'the' siren's call in manufacturing. Basically, price is about the strongest motivator for everybody in the manufacturing industry. We want to use HE sensors because they're cheaper. But let's turn this around and instead ask this; how would you (and modelers in general) view the trade off of saving 90¢ while increasing your risk of a crash due to the part failing suddenly instead of failing gracefully? Further to this; since some manufacturers use them, why aren't all manufacturers using them?

Naturally, I can't speak for other manufacturers - but - I have a theory. It has to do with trying to gain market share. Or haven't you noticed it's the no-name oriental guys offering up cheap servos with Hall Effect sensors instead of the big guys? So keep this in mind, we don't because of how HE sensors crap out. The newer players are just interested in a sale, like can you even buy parts for some of them? More later on this. Also, consider this; the risk is on you! And recall; we're not talking about just the risk of failure, but of 'sudden' failure, OK?

Anyway, I'd bet a milkshake most modelers would react like me to the thought of losing a model over a part costing about a buck. Frankly, I suspect you'd take a dim view of this principally because of 'who' bears the risk; am I right? So in theory, a Hall Effect sensor is better because it doesn't have a million-cycle lifespan (meaning it does wear out). And note; as long as it's working it doesn't matter what kind of pot you're using. But, in practice, is does matter because of how magnetic sensors fail when they do - suddenly. Puts me in mind of my favorite Yogi-ism . . .


Parts is parts:

Please allow us a bit of salesmanship regarding ProModeler; with other brands if a pot goes south you have to send the servo in for service because they give you no choice. That's too bad because replacing a potentiometer is a relatively easy job (with just three solder joints, it's a doodle). My suspicion is this is on purpose. Why? It's because by the time you're billed 1/4-hour for labor, plus the price of the part (and shipping both ways), few modelers will give the go ahead. Instead, they'll toss the servo and buy another. Coincidence? Follow the money!

Anyway, as modelers ourselves, and knowing many of our customers aren't afraid of a soldering iron, we decided to offer the pots alone. So beyond gears and cases (like the other reputable servos manufacturers make available), we're happy to offer the pots for those unafraid of DIY repairs!


Birdwalk:

Allow me a brief detour down memory lane by letting me tell you a story; back in the day it was routine to open your servos by removing a few small screws to reach the potentiometer. Once inside, loosening a couple of very small common-head screws permitted access to the delicate carbon wiper surface of the pot itself. Next, using a cotton swab and isopropyl alcohol you'd gently clean the carbon surface (and the wiper tip) thus, restoring performance to like new. And presto you were good to go for perhaps another year! I did it so often I could do it in my sleep and maybe it's because I've been doing this a long time, but these days, I prefer the ultrasonically sealed pots because they're just less trouble!


Fast forward to today (approaching 50 years of modeling for me, personally) and all available pots are sealed. They're also rated at a million cycles (not all, just the best ones like we use). But a million is not forever and thus, pots are actually a wear item. Of course the ginormous oriental servo companies know it. They could easily sell you just the pots - but they don't. Since we do, bear this in mind when next you're shopping for servos because it's the little things that make or break a relationship . . . and we prove it!

Anyway, and granted it's a small thing but in this way ProModeler is different. It's because we're not run by the beancounters, but by the engineers. And we're modelers just like you! The point being, we actually use our own product (in the trade it's known as eating your own dog food)

Thus, you can take it to the bank we 'always' try to make decisions with respect to reliability, centering, response, and accuracy with modelers foremost in mind. And it's entirely because we, too, are putting models at risk when we fly. So next time you throttle up your model to take flight, remember we're also doing it . . . just like you!

Recap:

Anyway, this is the story behind why our servos are using mechanical potiometers instead of electronic Hall Effect sensors. Yes, it would be easy to try and gain market share by promoting the long life aspect of a Hall Effect sensor, but as an engineering company first and foremost, we think it's malpractice to spec components that are subject to sudden failure when perfectly good slow-fail components are available. Especially in light of the potential consequences of sudden failure, capice?

In closing; it's about making the hard decisions and not just price. It's about watching out for our customer's best interest by using the 'how' regarding part failure as a factor in deciding what kind of parts to use. And this explains what we live for at ProModeler!

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