Linear optical encoder
Friday, November 13, 2009
Optical encoders are nothing new; they can be found in everything from mice to printers. They’re great for allowing DC motors to know their exact position and even current direction. If this is sounding like old hat, it’s because we’ve shown you rotational versions before.
[Chris] uses the same concept, but produced a linear optical encoder instead of rotational. His setup is much like whats used in non stepper-motor CNC and RepRap mills, allowing ordinary DC motors to know their position within a plane. It’s a quick tutorial, but we liked the detail and it reminded us we need to finish that DC motor based mill thats still a pile of parts in the closet. Check out a video of [Chris'] in action after the break. Read the rest of this entry »
8 komentar:
Anyone who wants to do this should look at the guts of almost any inkjet printer – the fine mylar band is one of these encoder strips.
It must be possible to do this with a measuring tape for greater accuracy.
That encoder looks ghetto…but if it gets the job done that’s what matters.
I have seen linear encoders quite a bit in printers that I have torn apart. They have a nice little clear plastic strip with lines on it a ‘pixel’ apart.
It’s a great hack, just to build something on top of this!
i wonder if he could achieve greater accuracy. as he is using PWM, i think he could. those mylar strips Karl mentioned are insanely dense, he would need an prebuilt encoder, something like this:
http://farm1.static.flickr.com/1/129350625_7ce76adffb.jpg
and of course a translucent tape.
Keep up the good hacks!
Something that they’ll have to be careful of (with only one sensor) is location accuracy.
Having more lines closer together will increase it, to an extent, but by increasing resolution.
Imagine the above system with very thin white stripes, same resolution but higher accuracy on hitting the same place irrespective of the approach direction.
Another way (in the absence of being able to make infinitely fine white stripes) is by off-setting another (identical) sensor (it doesn’t even need to be very close to the original one, it can be n-stripes plus the fraction away, it’ll only limit the ultimate travel). When the combination of the two sensors says “white stripe” then you know your location is within a stripe and not just over the end (as the other sensor would report “black stripe”).
One could use the analogue nature of the sensor to see if you are only slightly over into white from black but this assumes consistent behaviour on every white/black boundary and all operating environments.
Sorry, I went on a bit (I’m wasted on my day job when I could be hacking away on real electronics)
I’ve been toying with the idea of building a huge 2*2 metre x–y plotter, and a linear optical encoder would be perfect for this. But how can I get my hands on a 2 metre long transparent plastic strip with tiny lines? Any thoughts would be very welcome.
I have often pondered the problem of linear position measurement. I thought the best way might be to use some resistance wire and a wiper on the carriage. With using a straight length of resistance wire the impedance for a short run might be very low and so the voltage supplied across it would have to be low (so as not to melt it). It would be very cheap and absolute though, the positioning accuracy would be limited to the resolustion of your ADC.
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