Home made optical rencoder issues

[Macgman2000]

Oh been there done that. After a lot time in front of the scope I found the following issues when creating my own optical wheels.

1). photo transistor collector resistor if set too high will bounce output pulses. IE, it is too sensitive to ambient light. I suggest you take a black pen cap and poke a hole in the end facing the LED. This is just for a test to dial in the problem. Also try 4.7K~30K collector resistor using a 5v supply to stiffen the response to eliminate bounce. Try to change LED output level too. You will find through paper the difference between thresholds is very narrow, see point number 2 below if that is the case.

2). IR goes through paper even printed with black ink. The best optical wheel is one made of 0.020 black plastic or better yet aluminum with windows for stripes.

3). Try edge reflection rather than break beam. The problem will be how thin the stripes are on the disk. The finer you go the more likely you need surface mount parts with reduced photo transistor sensing area and narrower LED output. You will need a pin point light source/detector. I made quite a few with surface mount, the challenge is alignment affix all optics to the same board and space correctly.


Nick

[Elnino]

Thank you all for your input. It seems this was never going to be as easy as i initially thought.

I have not had much chance to play with the setup just yet. I have been concentrating on trying to print better disks. If the toner is totally opaque, it will block IR.

Alain you may use this method for UV exposure of your boards but UV is at the other end of the spectrum to IR. We can't really expect them to perform the same.

I have a 10k resistor on the collector of the transistor now, and as i have stated before, this setup works perfectly with other objects breaking the beam. No silly mass counts or bouncing of the circuit. The only other thing i thought of (and what nick just mentioned) was to reduce the intensity of the IR LED to see if it is less likely to penetrate any flaws in the toner (at least limit it enough to keep the transistor off).

Failing that, Although super awkward for something this small, i'll probably have a shot at aligning 2 disks as Al mentioned earlier.

I'm almost there - I don't really want to go to a full re-design of the sensors just yet...

[Elnino]

Ok, so i stuck one of my disks under the microscope and to be honest they look pretty darn good! The toner is solid and the edges are a little 'furry' but only about 1 micron (1/1000mm) is furry. I doubt many commercial encoders would be much better.

There were a couple of sub micron holes but this particular disk had not been cared for - it was one of my test ones. I doubt they would have any effect anyway.

So, where does that leave me? Well, it means doubling the disks is probably pointless but making the clear slots smaller might be of some help. I sincerely doubt that a 1 micron 'edge' is going to cause a problem for a sensor that is likely to have a beam width of probably at least 10 times that.

[Elnino]

Ok, so i've managed to have a bit of a play with my encoders - hardware and software and managed to get it 'smooth'.

I changed from disks with 6deg clear and 6deg black to 2 and 10deg. This made a big difference but did not resolve the problem.

I also printed these disks in colour but they are still black. The PS file i have for making the disks orgininally printed black. I changed it to print colour but all levels are 0 so it prints black but with the CMY colours. This makes for a much denser print.

On top of that, a simple 2ms pause at the top of the ISR before reading the port gave it sufficient time to settle in either an on or off position.

Problem solved! Thanks to all that helped me out.

Andrew