Home made optical rencoder issues

[aratti]

Without the chopper disks, if i break the beam on the sensor with something opaque, i get an increment of 1 on the counter and same when the beam returns to unbroken. This works as expected. If the sensors are left alone, i do not get any increment. With the encoder disks in place i can rotate them at pace and get believable increments. My encoders are ~50 pulse/rev and i can get about 50 pulses if i manually rotate it about 360 degrees.
My problem is when i rotate them slowly - i.e just a small movement might increment the counter by 10 or 20. My assumption is that the sensor is on the edge of the opaque bit on the encoder and 'flaps' a bit. But, i do not get this when i manually break the beam which seems odd.

That simply means that your disk is not sufficiently opaque. Tray with two disks one on top the other (even three if you have room), this will increase the opacity by a factor of 2 or 3.

Al.

[Elnino]

thanks for the input people, i really appreciate it.

I am going to try printing them on another printer first. The other one i have is a Xerox which has the ability to 'pile on' the toner and make it really thick. The other hp laser i have is a little on the pathetic 'economy' side.

The awkward thing is, the data sheet does not really give specifics on the working area of the sensor, only where the optical center of it is. i.e it does not tell me the minimum width over the optical center to sufficiently break the beam.

I think i should be able to get it to work without any extra electrics or de-bouncing but it might take a bit more trial and error. I don't think its a debouncing issue as it does not bounce when i break the beam with something random - even a piece of torn off paper seems to be fine... It mush just be my dodgy disks.

[boroko]

I remember reading somewhere that someone was having similar problems with printed disks. The issue was that since the detection was IR, the visible black was not being seen by the sensor. You might try something else for those detectors as a test, say stripes of black tape.

Just a thought.

Bo

[Acetronics2]

Hi,

Did you had a look to Parallax kit of 44 " stripes " wheel encoders ???

http://www.active-robots.com/product...lwatcher.shtml

look same as you try to do ...

Already working.

my two cents...

Alain

[Elnino]

I remember reading somewhere that someone was having similar problems with printed disks. The issue was that since the detection was IR, the visible black was not being seen by the sensor. You might try something else for those detectors as a test, say stripes of black tape.

Just a thought.

Bo

Yeah, i had thought of that but the size of my disks are only about an inch round OD - Pretty hard to do something like that by hand. Perhaps there are microscopic holes in the toner still letting the light through. I might pop one under a microscope at work and have a look. I also read somewhere else that when printing these, it can be better to print them on a colour laser printer but using CMYK to print it and not just the black. The theory there is more toner on the surface and it's more likely to block the IR. I might also give that a try.

Hi,

Did you had a look to Parallax kit of 44 " stripes " wheel encoders ???

http://www.active-robots.com/product...lwatcher.shtml

look same as you try to do ...

Already working.

my two cents...

Alain

Great find - i looked all over for something 'pre-fabbed' that was not too exensive but didn't find anything. Had i come across them before i might have gone that way, however i've now built the 'sensors' so i'd like to persevere with them for the moment.

[Acetronics2]

Ok ...

So ... The first good thing would be tho have access to the base of the phototransistor ...
just to make it less sensitive ... ( you've correctly read ).

Second ... SPEED ! Opto transistors really are lazy components ... I just dig into my docs to find an old 30 Years old Motorola scheme intended to make performances acceptable ...

Also consider Honeywell HOA 901 or equ. components ... worth the try.

Third: encoder Wheel ... did you try the transparent Polyester film for Inkjet Printers ???
... that's what I use for my PCB UV Exposure !!!

read you soon

Alain

[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