Very slow lazy susan

Hey guys, I'm helping my fiancé with her solo art show and she needs to move a GOBO projected light across a wall at a very, very slow pace, so that it takes about 8 hours for it to travel across the wall. Now we'll be using a gobo projector that will be sitting on a pedestal, so I was thinking I could create a motorized lazy susan to rotate it, but I wasn't sure if I could use a normal servo motor at such slow speeds or if I'd need to use a stepper motor. Servo would be preferable so the movement is smooth, but if that's not possible we'll have to make do.

Apologies if this is a dumb question, I'm a programmer by trade but this is my first foray into the engineering side of things. Thanks!

A servo makes steps of 1/180 when the Arduino Stepper library is used. It doesn't seem the right choice for this task. The light would move once in a while and that would not be a smooth movement.

A steppermotor with a gear seems a better choice. Or a normal motor with a gear, if you have some kind of feedback on the position.

Unfortunately we wouldn't have any positional feedback, so it sounds like a steppermotor with a gear to smooth it out would be better.

A stepper motor should be fine. The important thing is the gearing between the motor and the lazy susan so that many revolutions of the motor produce a fraction of a revolution of the lasy susan.

move a GOBO projected light across a wall at a very, very slow pace, so that it takes about 8 hours for it to travel across the wall.

The needed information is the throw-distance, the width of the projection, the total wall/screen size... that is, you could move a projection on a fixed width screen with new scenes coming in from the left-side OR you could move the entire projection of x-width across a larger wall of y-width.

The next piece of critical information you have provided, 8 hours. Unless this entire period is being video taped, I doubt that anyone would be observing the scene for the full 8 hours. So, small steps in movement would likely go unnoticed BUT this needs to be confirmed with the fiancé because of the long-term potential impact of a poor decision on your part. :roll_eyes:

I'm not convinced that smooth movement is the right choice, it certainly complicates things to some extent unless you plan on building your own custom motor reduction gears/pulleys and such. Knowing the throw distance and the stepper motor steps to cover that distance easily makes for a simple timer in software to racket the platform.

Ray

this needs to be confirmed with the fiancé because of the long-term potential impact of a poor decision on your part

Without a doubt - test the sytem well in advance of the launch.

So, small steps in movement would likely go unnoticed

You need to know the projection radius to determine how much the image will move. However without gearing I would expect the movement to be noticable, its not 100% clear if this is important or not.

Erdin: A steppermotor with a gear seems a better choice. Or a normal motor with a gear, if you have some kind of feedback on the position.

Yes - or perhaps a DC motor without feedback, if the mechanical resistance is constant-ish and you are not concerned about maintaining a precise speed. (Once you have selected the right motor and gearing to get the speed about right, you could use PWM to slow the motor down to fine-tune the speed.)

I would experiment with a servo before trying anything more complicated. If you use servo.writeMicrosecond() you should be able to move it in very small steps.

...R

Thanks for all the help guys, this should give me a good starting point.

Have you thought about an AC mains synchronous motor? Some are made (with gearing) to turn at extremely slow speeds. The ones used in time switches turn at one revolution per day. That's 360/24 =15 degrees/hour giving you 120 degrees of movement over 8 hours and without any jerkiness (is that a word?). If that's too fast for your requirements, you could use your arduino to periodically cut the power to the motor. Short periods (a few seconds) of inactivity would not be noticeable at that (lack of :)) speed.

The ones used in time switches turn at one revolution per day. That's 360/24 =15 degrees/hour giving you 120 degrees of movement over 8 hours and without any jerkiness

Similar applies to the hour hand on an old fashioned mechanical clock. The tick-tock drives the hour hand 360 degrees in 12 hours so it will cover 240 degrees in 8 hours.

It might be possible to use a mechanical clock itself, or use the mechanism as a ready made gear system to which you could attach a stepper.

You would need to ensure that the motor can supply sufficient torque to overcome friction and the inertia of the lasy susan, and that the gear mechanism can withstand the extra strain.

You cannot see the hour hand of a clock moving, but that is just because the arms are relatively short. You do need to know the distance you are projecting over and the number of degrees you will rotate in the 8 hours to work out what the visual effect will be.