I was reinstalling encoders today (after having made new mounts for the gearboxes), and saw some specks of dirt on the encoder wheel.
These are the 360 CPR, 1/4" bore E4P encoders from US Digital.
I was wondering how much dirt you can have on the encoder before it impacts the reading. How does the phototransistor look at the wheel? Does it look at the whole of the line, or just at a single radius?
If one of the encoder lines were to be blacked out, would the quadrature decoder miss TWO whole counts? (Is there enough space in between the two phototransistors that it would miss the count when the blacked line went under channel A, and again under channel B?)
An interesting thing to note about the wheel is that it’s fairly scratch resistant. When I was taking a wheel off, that plastic ring (with those lines) came unstuck from the aluminum. It’s a thin piece of plastic, but the black lines are on the sticky side up against the aluminum, so to not be scratched off by the wheel being too close to the LED/phototransistors. It wouldn’t surprise me if a little speck of dirt or grease got trapped under the plastic ring when I stuck it back on. Is this something I can clean, or will that harm the adhesive or the lines?
Marshal,
Dirt can cause an error albeit a small one if it stays with the wheel and doesn’t jamb under the emitter or receiver permanently. If it amounts to one or two errors per rotation then you still end up with 350+ good data bits per rotation. You should try to keep debris away from the encoders and cap them after you mount the wheel to the shaft.
I think the dirt would have to be fairly large to fool the device though.
This year at FTC, our robot couldn’t run straight. At first, we thought one of our motors was weak. So we replaced both of the motors. It still couldn’t run straight, so we finally took one of the encoders apart and examined the optical disc, it had a minor scratch on it. Apparently, one of our students did not use the spacing tool when putting the disc on. After replacing the disc, the robot can run straight again. So it is critical the disc is free of scratches or dirt. However, if it is really dirt on the disc, can’t you use a soft damp cloth to clean it?
I found out otherwise:
There are large nicks in the encoder surface, probably from the wheel being improperly removed when the gearbox is taken apart.
We will probably purchase extra encoder wheels from US Digital. I believe they’re only $12 apiece.
Yep, make sure you use the spacer tool when installing the disc. Our student pressed it all the way down without the tool and that was enough to scratch the disc. Fortunately, we have some fried encoders so we swapped their disc.
Installing it has been pretty easy. Except that the CIMple shafts are slightly undersize. My solution is to bend the tabs on the encoder in slightly.
Removing it has been the hard part. Aside from getting the cover off in the first place, I’ve had difficulty pulling the encoder wheel off. Is there a tool for this?
No, we just use our fingers slowly and carefully pull it off. If it’s really stuck, you can probably use leverage of any small tools but make sure the tool has no sharp edge and use something soft to cover it (e.g. cloth).