How should I drive a curtain cord with a stepper motor?

[Graeme]

I have a corded curtain track (http://www.polesandtracks.co.uk/acatalog/ccpt-crdlstylealm.html) where each curtain is driven independently with a cord at each end.

I'm trying to hook the cords up to stepper motors with a 5mm shaft, but am struggling to find the right way of going about it.

Some ideas that I have:

• Use a v-belt pulley and line it with a high friction material (e.g. sugru/grip tape/small, wide rubber band/etc)
  
• Make my own pulley using a 3D printer or thermoplastic and maybe add some friction with multiple windings or a rough surface
• Use a wide timing pulley and line it with a high friction material (I might even be able to get a couple of windings if it's wide enough)
• Something like this roller wheel (again adding a high friction material)

Does anyone have experience driving cord with a motor, rather than driving a belt?

Does anyone have some other/better ideas of how I can transfer power from the motor to the cord?

[Dennis Smith]

I'm not 100% sure if in your case this will work, however I wonder if
you can put knot in cord at end and have it tied to centre of wheel
driven by motor. The result is it will pull as much as you need, then
release when needed. perhaps 2 steppers working in opposition to each
other to release other side of cord. This assumes that the cord is
long enough and doesn't require it to be a loop as some do. As one
motor turns clockwise the other does the reverse, so one cord is
pulled tight and the other is released.

Just a thought that removes the friction issue altogether.

Dennis Smith
M1DLG

> --
> You received this message because you are subscribed to the Google Groups
> "Reading Hackspace" group.
> To unsubscribe from this group and stop receiving emails from it, send an
> email to reading-hacksp...@googlegroups.com.
> For more options, visit https://groups.google.com/d/optout.

[Matthew Beddow]

Is there a way to eliminate the chord all together? In most bilnd etc, pulling the chord rotates a wheel which drives a shaft that moves the fabric. You may be able to directly attach the stepper shaft to this shaft, assuming it has enough torque.

[Jay Abbott]

If you make the diameter a bit larger and wrap the cord around it a
few times, you could probably use any relatively grippy surface on a
cylinder/wheel/pulley, for example an old inner-tube cut up and glued
around an old deodorant can with a 3D printed gear mounted on the
bottom.

[daprigoo]

3D Printers which use spectra line usually have a hole in the shaft or
in a pulley to secure the line as the shaft rotates. Something to get
your started - http://www.thingiverse.com/thing:36951

David

[Andy Noyes]

Yes, I've done something similar, but not with curtains. My solution was to use a drum, attach both ends of the cord/cable to it, wind one clockwise and the other counter clockwise. As the drum rotates it pays out cable one way and reels it in the other way. If you want to get more advanced you can cut a groove in the drum to guide the cable. Obviously the drum needs to be sized to hold enough cable for the maximum travel you expect.

[Norro]

yup this drum method is what I was thinking

[Eric Rowen]

I suspect that the cord is in effect a continuous loop, in as much as the two cords you see on the diagram are the top end of the loop.   This "loop" is normally fed round a spring loaded pulley mounted on for example the floor.  So  pulling on one side draws the curtains and on the other opens them. 

This is not to say that the loop could not be cut so that combining ideas (the knot) plus one cord wound clockwise and another counterclockwise round a pulley would work well.   

However simpler would be to just wrap the loop several times round a pulley to gain sufficient friction.    Tensioning the cord as in the manual operation could be achieved with a spring.  Either using an idler pulley or (more difficulty)  spring loading the whole motor and pulley assembly.   Again from using the manual setup some experimentation may well be needed as pulling on one end of the loop can cause the other side to go slack.

An option here would be to use the original tensioner mounted say on the floor and site the motor/pulley part way up and just wind the cord several times round the suitably sized pulley (covered with some suitable friction material)  This would then combine the manual tensioner which would help the cord grip on the pulley with the motor control

Gearing or a large pulley combined and/or with quite a powerful motor may be needed to pull the curtains (from experience of having these type of curtains in the past (manually operated) 

If there are two curtains then I do not see any easy way to operate both with one motor (this was not implied in the question but just to clarify things) as the cord (loops) will be at opposite ends. 

--