Counting the length of a string with knobles

[crazeuk]

Hi all. I am fairly inexperienced with arduino.

I have a challenge i have been set.

I need to measure the length of a roller blind chain, like this:

Which will be automatically pulled through the counter to a set length (entered through a key pad and displayed on a screen, then cut.

It needs to measure in mm, and needs to be accurate 0.4mm (diamater of a ball on the chain).

I can use the existing roller mechanism like:

To turn a shaft as its fed through.

I was thinking of using a setup like the old ir encoders from a PS2 mouse for the job (attach the encoder dial to the shaft). As the shaft turns the arduino would pick up the breaks to measure how many it had to work out a length.
Then a solenoid would be triggered to cut the chain.

Would i be correct in the approach?
Should i use a salvaged sensor from a mouse with chip or can i buy a it speed sensor and use that.

Thanks

[Railroader]

Attach an encoder to the axle. I suppose You don't want to cut through any ball.

[madmark2150]

You should be able to use an spoked/cogged wheel and an optical interrupter to count the beads as the wheel rotates. This should give you "to the bead" equal lengths just by counting. As long as the wheel can't slip, you can just count the tips of the wheel going by at any point in the rotation (you don't have to directly sense the bead, the wheel stops turning if there are no beads.) This makes it mechanically simpler as well as trivial to code.

[octopirate]

@crazeuk

needs to be accurate 0.4mm (diamater of a ball on the chain).

Diagram shows diameter as 4.5mm....

[madmark2150]

Prolly meant the link space between balls.

[octopirate]

That's 1.5mm if the diameter is 4.5mm and centre-to-centre is 6mm.

[madmark2150]

Prolly a mistake then. Doesn't change the basic idea. This is where a 3D printer comes in handy for making one-off proto parts.

[flashko]

Isn't it easier to use a stepper motor with the white wheel in the picture? The wheel has 12 sockets (I guess) and this means that 1 revolution is 12 balls, ie. 72 mm. This makes it easy to program how many steps to turn the stepper motor to the desired length. Perhaps it would be good to set the desired length to be a multiple of the string step, ie. 6 mm. I don't think there is a need for additional sensors and the program becomes quite simple.

[jfjlaros]

I would use a scale.

[madmark2150]

Depends if you're driving or driven. I thought the idea was to count as something else drove the spindle and you needed to fire the "cut" solenoid every Nth bead. Naturally if you're DRIVING the feeder, then yes, a stepper, or even a solenoid as the pawl on a ratchet, makes more sense. But if someone/thing is pulling the beads and we need to say "enough", then a sensor is needed to watch the idler turn.

[Paul_B]

@crazeuk

Should i use a salvaged sensor from a mouse with chip or can i buy a it speed sensor and use that.

Either way, your challenge is to mechanically interface the rotary encoder with the existing pulleys.

In the "old days", we would have got to work with a lathe. Nowadays we are likely to design a part in software and print it.

Depending on just how much this will be used, bearings and wear may be a problem

[crazeuk]

@madmark2150

Doesn't

Need to double check the chain, although the 4mm was me taking a rough guess as to the tolerance requirements

[crazeuk]

Thats exactly my plan . I thought let me get the electronics sorted and the rest will fall in place.

[crazeuk]

I actually never even thought of that. Thats a good idea!

[crazeuk]

To be honest with you, whether its driving or driven i didnt consider.

My initial plan was to use something like this, i hacked together of an old blind:

The rod would have been attached to the slotted wheel for the senor.

Then to have something else rolling / pulling the chain through.

[acjacques]

If the knobles are made of a ferrous material you may consider use an inductive proximity sensor.

[bluejets]

Print up a lightly spring loaded tapered end lever with a slotted optic switch off the same.
Count the times it drops into the joiner section and when count reaches the required total, "CUT".

Above could be located anywhere along the chain so long as an arrangement is made to keep it in a guided path.