Thread: Cheap encoders
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Unread 26-12-2008, 13:43
Ivan Helmrich's Avatar
Ivan Helmrich Ivan Helmrich is offline
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Re: Cheap encoders

I too purchased a few of these. I really don't understand the quality concerns people are expressing. While this exact model number isn't available, other comparable models of the same brand are available from Mouser Electronics for about $51.67. Not a cheap part at all. Mechanically, they seem more robust than the plastic housed US Digital encoders I have. The published specs seem to be comparable to the popular US Digital part as well. (I'm not knocking US digital, I like their parts and will almost certainly be using some this year.)

It's true, they have sleeves and not ball bearings. This doesn't reflect quality as much as intended purpose however. Sleeve bushings work just fine in the right application.

So, what application? With a life of 10,000,000 rotations at 300 rpm, I get something like 550 hours of operation, or about 16000 FRC matches. Our robot won't see that many. 300 RPM is a little slow for a drivetrain but they should be fine for just about any other lower speed application. I'm pretty sure I'll be using these somewhere this year too.

So, at $4 each, I couldn't resist taking one apart to see what is inside. I slipped the snap ring off the shaft and gently pried the back cover off. It came right apart with snap. The disk is what appears to be etched stainless and the circuit board is a simple affair without any screws to come loose. The drag on the shaft is due to the grease used and the good fit of the sleeve and shaft. So this leads me to another thought.

At the most, I think our robot could only see about 100 matches. More likely, fewer than half that. So now, I can think about de-rating. If I put these on my drivetrain, they might rotate up to 500 RPM or so. Not even twice the rated speed. I'm willing to bet that this won't reduce the life by a factor of 160, so I should be good to go.

The only issue I see is if as mentioned by Gdeaver, the wave form goes soft at higher speed. I'll bet not, but I'll put a scope on one to make sure.

Would I use these on the drivetrain of a bomb disposal robot at similar speed? No way! Lives could be at stake. To save a few bucks my team can use elsewhere? You bet. It's a good exercise in value engineering. YMMV
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