Hi, Arduino newbie here. I'm trying to build a light fixture similar to this:
I'd like to raise/lower the 2 wooden rectangles independently. I imagine doing this with 4 stepper motors per rectangle. I have two questions on that:
Is there a standard way of attaching steel cable to stepper motors or do I need to rig something myself?
How do I determine the type/size of stepper motor I need, given X lbs and how quickly I'd like to raise/lower?
Tips any any aspect of the project are also appreciated. I'm planning to put the lights on 2 separate circuits with relays (or maybe TRIACs for dimming) so I can control them via Arduino as well. Thanks.
I think you should look at using a winch.... well that's what you're trying to male I suppose. You'll need a reel to take up the cable as you raise the lights.
I'd investigate off the shelf winches if I were you- I'm sure makers' spec sheets will help you select the right ones- then sure, think of controlling them with an Arduino.
devth:
I'd like to raise/lower the 2 wooden rectangles independently. I imagine doing this with 4 stepper motors per rectangle.
That sounds like a bad idea for a start. I can just imagine the steppers getting out of step and the thing hanging askew. If you imagine the winch mechanism being so compact that it can be part of the visible fixture, you are looking at some serious design and manufacture expertise; I think it would be more practical to have one winch (for each part of the fixture) concealed elsewhere with a span of winch cable facilitating the split to four (or preferably, three) cables running through pulleys.
Clearly you require a winch drum. If you had it hidden in a wall cavity, you could have only three or four laps around the drum and a counterweight to hold the remaining cable. Whether you use steppers or windscreen/ car window motors (more practical), you need a "limit" mechanism which may simply be running against a hard stop (and detecting the stall current if using the DC motors).
I would suggest attaching all the lines to a common pulley, and design it so that the pulley physically can't unwind further than it is safe to. Then turn the pulley using a suitably rated motor and gearbox with an encoder.
Sounds like a common pulley is the way to go. Don't understand how one would prevent the pulley from unwinding. Is there a specific type of pulley that enables this?
devth:
Sounds like a common pulley is the way to go. Don't understand how one would prevent the pulley from unwinding. Is there a specific type of pulley that enables this?
No, I mean a pulley that has the cables attached in such a way that trying to unwind the cable past a certain point won't let out any more cable and won't result in the cables becoming detached from the pulley.
Just drill a hole through the diameter of the pulley, stick the cable through, and knot it. It can never "unwind" that way. Granted you can't knot steel cable but you can put a ferrule or something on it.
When you're rigging this up, remember that to keep the rectangle level that all of the cables have to be the same length between the pulley and the rectangle. Putting that another way, if you put the pulley dead center and all of the cables attach to it then when you unwind the pulley the cables attaching to the long sides will go slack and the short sides will bear all the weight.
If it were me I would use a long dowel as a pulley. Run the dowel parallel to the long sides of the rectangle but off-center of the short sides. Two cables can then run down vertically to the short sides with the third cable running horizontally over an idler pulley and then vertically down to the "far" long side.
As an addendum to that, instead of four small steppers, it would be simpler and cheaper to just use one large stepper and stepper driver. Also note that you can't run multiple steppers in parallel off a single driver or everything will go out of whack.
Once you have worked out how you are going to do it !! I would use the Big EasyDriver module from here
Which you can source through Sparkfun.
I would do each of the frames with a reasonable sized stepper motor and gear it down as required using worm drive.
have a look at a recent thread on here about panning a twin axes camera to see what you can do with some simple meccano parts - using a worm drive dramatically increases the power of a motor.
If you did not want to use a Stepper - then i would definitely go for a car windscreen wiper motor as these have massive amounts of torque available. You would then have to use some form of off board distance limiting - and as other have said a fail safe where the cable was connected to the drum (although personally if it was me i would have a seperate stainless safety harness)
I would use the Big EasyDriver module from here