Stepper motor power help

Hi Michael,

A stepper motor isn't needed for this, if you want it to perform constantly. You could just use some mechanical switch to know when to turn the motor off. But, the jerky action of a stepper motor will be an advantage at Halloween : )

The tiniest motor can be used in theory, if you want a slow speed, with just very high reduction ratio with your gearbox.

If you know the diameter of the pulley/fishing line reel, you can calculate the torque, and then divide it by your gear ratio, to find out what torque motor you need. The same applies for a stepper (and you might need less or no gearing)

depending on the size of your final choice motor.

This link and the wikipedia entry will be useful for your calculations. You might need to convert units, since motor torque may be specified in Nm or mNm.

I agree with shabaz that a stepper is the wrong motor.

There is a another way and that is to use a weight and pulley.

You wrap the line around some form of pulley on the motor and this then controls the direction, but only has to do part of the work.

The speed remains constant as the pulley size never grows.

mark

Thanks for the answers, but let me further clarify the problem.

Constant refers to not letting the motor rest, but it would be letting the object (pumpkin) lower approximately 6 feet, then reverse direction to raise up 6 feet, then reverse again.. etc.

Also due to the need for invisible support, I plan to use a very thin fishing line. Not sure, but I would suspect there would be slippage on most pulleys.

So.. what size stepper can lift about 2 lbs by winding line on its shaft?

Michael,  as suggested could you use two pumpkins, so they counterbalance each other.  The fishing line is fine to carry the load but being nylon it is essentially a bearing with low low friction characteristic.  I think the jerky motion of a stepper motor would exacerbate the problem.  I think you need to look at a pulley on the motor shaft with a high friction to the nylon fishing line and multiple turns of the line around the pulley.  Essentially you need to grip the line.  Possibly a pinch roller to increase the grip, a rubber tyre (UK spelling) would also help.  The larger the pulley diameter and the slower the motor will give more chance of sucess.  Another suggestion is to coat the pulley with a fine abrasive paper.

Hope this helps.

Actually I was going for 3 pumpkins. But I think 2 in perfect opposite balance would not look quite right. I want 3 random up and down, not in sync. And with the counterbalancing you suggest, what would cause the motor to switch directions?

Also from the vids I've seen, I was under the impression using a driver would smooth out most of the jerky motion.

what would cause the motor to switch directions?

same as the stepper, the program instructs the direction change.

I was under the impression using a driver would smooth out most of the jerky motion.

Your impression is incorrect.

If you don't want to use pumpkins, you could simply use a weight.

Mark

You can use a stepper to do what you want with some hard mechanical work and paying attention that it works rotating some specific angles ( no continuos movement).

To control it you need some electronic parts (for instance arduino and L293D)

I am supposing you have something weighting 2 lbs (aproximated 1 Kg) suspended using a rope and connected to a wheel of a defined diameter (for instance 10 cm = 0,01m). To lift that piece is required a binary of 0.01 Kgf.m that corresponds to 0,098 N.m. Than you need a stepper with a capacity higher than 0.1 N-m.

Now is to choose one stepper according your other parts of your project.

@Fernando - some maths errors have crept in !

10cm = 0.1m but that's the diameter so the radius is 0.05m.

The 1kg load would be about 10n so the minimum torque needed from the motor is 10 * 0.05 = 0.5Nm.

The OP wants to lift the load about 2m so that would be 2/(0.1pi) = 6.4 revolutions of the wheel.

MK