Stepper motor advice

[InfernusDoleo]

Hi all - I have a question and I'm almost completely out of my element here.

I'm an amateur halloween prop builder. I've an idea for a prop and mechanically, I cant make it work. But then I thought, if I use a stepper motor, then I can control the exact distance the motor turns, and it will allow me to build the prop correctly.

But I know almost nothing about stepper motors. I'm here because I own a laser engraver/cutter, as well as I'm in the signmaking business and own vinyl cut/plotters. I also know a hell of a lot about IT/programming, and last year I built some pretty cool props using an arduino mega board.

So, I have background.

I just dont know stepper motors.

Where should I look for an affordable stepper motor that can handle a little bit of torque? A few pounds anyway. Needs to handle more than a servo would, but we're not talking 15 pounds of lift. It doesnt need to be crazy accurate - I just need to be able to tell a motor to turn 5 times clockwise, then turn back the same amount.

Here's what I'm building, maybe someone can reccommend something better?

I'm building a scarecrow, that will lunge forward (gravity) on a hinge on it's post. After it lunges, a motor needs to wind it back into position. The problem I'm having is after it winds, I need to disengage the tension so it can fall again. I have a few ideas, but they are clumsy.

If I can fine-control a motor to wind until the 'crow latches secure, and then unwind the cable the same amount, problem solved. Thats where I thought stepper motor.

Is that what I need? Is there another motor I can fine-tune control like that? Where can I purchase one thats not crazy expensive?

And if I understand correctly - they work like servos, correct? Or will I need something heftier than an arduino to control it?

Thanks for ANY info - I want to start the build, beings theres 3 months til halloween. But I need to be prepped.

[doorknob]

It seems to me that a stepper motor may not be the right motor for your application. You are not looking for precise positioning. Rather, it seems that you could get by with a reversible DC gearhead motor (driven by the arduino using an H-bridge circuit), a position encoder (to count revolutions of the motor shaft in either direction in order to verify proper operation), and perhaps some kind of limit switch (to detect when the scarecrow is in its home position, and maybe another one to detect when the motor is completely retracted - see discussion below).

The position encoder need not have great precision, either. Something like an optical interruptor that gives you one pulse per revolution might be sufficient.

I'm not sure that a wire or string-based tensioning design (if I understand your plan correctly) would be the best choice, though - have you considered using something like a leadscrew driving a nut (that pushes on the end of a lever that pivots about a fixed fulcrum) to move the scarecrow into position (where it would presumably be latched into place via a solenoid or a ratchet-like mechanism that will later be used to release the scarecrow to the force of gravity), and then withdraw the leadscrew nut after the latching is complete?

[InfernusDoleo]

I just figured a stepper motor would be the least amount of work... the least parts necessary. I'd like to fully explore all options though... can someone point me in the direction of some affordable motors, step, or DC gear driven, or otherwise? I'm not even sure where to start looking.

[doorknob]

DC motors:

MPJA - Power Supply, Power Supplies, Security Cameras, LCD, Fans, Etc.


Stepper motors:

MPJA - Power Supply, Power Supplies, Security Cameras, LCD, Fans, Etc.


Another source:

Motors | AllElectronics.com


and another source:

Motors-The Electronic Goldmine

[InfernusDoleo]

Sweet, thanks. Its not that I cant google - I just dont know where to go to to find the RIGHT motor thats not some industrial grade pricing

Now here's a noob question... How do I read torque? I know what torque is, but not how it's measured. For example, one motor I just glanced at said it had "6oz/in torque load"

What exactly does that mean? Obviously torque changes with gearing, but what exactly does 6oz/in mean? I can figure out all the conversions once I know where to start. Thanks for indulging my motor noobness

[doorknob]

A common unit of torque measurement for small motors would be "ounce inches" or oz-in. (Not ounce/inch)

You will also sometimes see small motors rated in kg-cm (kilogram centimeters), or in N-m (newton meters). There are simple conversion formulas between the different units.

The torque values are typically described in terms of force operating over a lever arm distance.

The torque rating of a stepper motor is usually given as a "holding torque" against which the motor will remain stationary, along with a dynamic curve that may show the available torque as a function of the number of applied pulses per second (for a given voltage and current), for example: http://www.kelinginc.net/KL23H284-35-4BT.pdf

The stepper motors typically used in hobby-level CNC machines often have torque ratings in the approximate range of 100 oz.-in. (for a very small machine) to 1000 oz.-in. (for much larger machines).

To get a better feel for what those numbers mean, here is a good article that gives hands-on experiments for measuring motor torque:

Measuring stepper motors

[InfernusDoleo]

So from what I'm reading, the holding strength of a stepper motor - if I purchased one that read 48oz-in, that would mean that if a 1" radius arm were attached to the motor, it could hold 3 pounds (48oz) before it would turn? Is there a way to translate that to pulling strength? Thats holding strength but what about the ability to pull that much?

I'm assuming torque would be read the same way... a DC geared motor that was listed as 4 foot-pounds would be able to lift 4 pounds with a 1 foot long radium arm? Am I reading/understanding that correctly?

[doorknob]

Although I have not performed such tests myself (well, maybe I did in high school, but that was a long, long time ago), that would be my interpretation.

As far as moving from holding torque to dynamic torque, look at the chart in my earlier post for an example. Stepper motor torque is generally greatest at lower speeds, and it falls off as the speed is increased. Each motor will have its own torque curve, but that one should give you the general idea.

That particular curve was for a motor with a rated holding torque of 387 oz-in. (from http://www.kelinginc.net/NEMA23Motor.html ) - note that the curve starts at 2 N-m and goes down from that. 2 N-m converts to about 280 oz.-in.

I used the conversion calculator at http://www.convertunits.com/from/oun...o/newton-meter

[James Newton]

So from what I'm reading, the holding strength of a stepper motor - if I purchased one that read 48oz-in, that would mean that if a 1" radius arm were attached to the motor, it could hold 3 pounds (48oz) before it would turn? Is there a way to translate that to pulling strength? Thats holding strength but what about the ability to pull that much?

You have holding torque nailed. Just in case you need to, here is a page on how to measure it:
techref.massmind.org/techref/io/stepper/measure-torque-rb.htm

The trick with pull-in torque is that it depends on speed, so it's really difficult to find from just the holding torque. You need to know not only the detent torque (which is often not rated) but also how fast you want the motor to be turning. Some motor mfgr provide graphs that show at what torque the motor will stall for a range of speeds. Note that these graphs are dependent on the driver and power supply as well.

There is a calculator on this page that will help you figure out the motor rating you need in order to move a load on a lead screw (which may not be how you want to set this up): techref.massmind.org/techref/io/steppers.htm#Estimating

I'm assuming torque would be read the same way... a DC geared motor that was listed as 4 foot-pounds would be able to lift 4 pounds with a 1 foot long radium arm? Am I reading/understanding that correctly?

I believe that is correct. And I really think that doorknob is right: You need to be looking at a gear motor with a very simple shaft encoder: A disk with an LED and photodiode or even a cam and a microswitch... or better yet, a limit switch to shut off the motor when the scarecrow is retracted, and a timer to run the line back out more than it needs to be. You can always have a second line to stop the motion at a preset distance. The retraction line can be longer than that with no harm other than a slight delay while the excess is taken up before the scarecrow actually starts to retract.

It's not so much the complexity of a stepper system that makes it unsuited to this application, it's the cost and the lack of torque. Steppers are best for relatively light, very precise, and complex movement.