Stepping Motors

Hi,

I have Stepping Motor and a controller which has variable speed and a 'Change Direction' push switch. I want to use it to rotate a light back and forth at any required speed, without actually having to stop the motor (simply by pressing the Change Direction button during constant operation, when required).

Are stepping motors designed to be run this way or is it not recommended? I read that with a 'normal' motor you wouldn't do this due to potential damage and lots of sparks, but wasn't sure about stepping motors.

I want some opinions from others on this.

Thanks.

Jason. Hobbyist

Reply to
Jason S
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Hi, Jason. If you're rotating your stepper at less than 1 rev./sec., and your stepper is lightly loaded, it might work.

You're right -- with a "normal" motor (i.e. DC PM motor), you'd get a real light show at the commutator, and might smoke the drive. Steppers don't work that way, though. The worst that can probably happen is that the stepper will "lose step". That is, the torque will oppose the change in direction, and cause the stepper not to rotate to the right position. Usually you can hear this happen -- the stepper makes a fairly loud sound as it tries to hunt towards getting in step again. This is more of a problem as you go much past 1 rev./sec. or so, and becomes much more of a problem with the greater rotational torque a rotating load on the shaft presents. When you're unloaded or lightly loaded, the moment of inertia is basically just the motor itself.

This also may put extra stress on any diodes in your circuit which are made to dissipate the inductive kick, but they should be OK. A good stepper driver can't be smoked that way.

So if you're working at slow revs and your motor is lightly loaded, you might just want to go ahead and try it.

Good luck Chris

Reply to
Chris

Hey guys,

Just wondering, given the Chris's explanation of the inertia problem: if the lamp is fairly heavy, perhaps you could get less mass on the shaft by mounting the lamp on something else and just mounting a mirror on the shaft--aim the lamp at the mirror.

Torben

Reply to
Lars Torben Wilson

I wonder if maybe a hobby servo might work for this. They are cheap, though not as cheap as surplus steppers, and many cheap and easy to build circuits can be found on the web to control them. Since they have position feedback the lost step problem is avoided. Another solution might be to go to a store and look at how a fan oscillates with only a mechanical linkage. Then the motor always turns the same way. This way a stepper could be used with the driving circuit controlled by a pot. Since you already have the stepper driver this might be cheapest. And stepper motors have higher torque at slower speeds. Good luck. ERS

Reply to
Eric R Snow

Thanks for all the replies guys, I appreciate it.

Just to give you an idea of what I was originally planning to do, I was thinking of using a small plastic case with a large LED bulb ... pretty lightweight really... the whole box would tilt at various angles by the use of a low RPM motor of some sort - thinking a stepping motor might do the job. I ran a couple of tests with one and found that it may fail on me at higher speeds.

So I've been thinking about a Servo or Reversible Gearhead Motor. I've used neither of these before, but looking into how they work. They may very well be what I am looking for, as I am trying to avoid the use of mirrors and potential "slips" and freezes that may result from a stepping motor.

Unless there are stronger stepping motors out there? haha

Thanks again,

Jason.

Reply to
Jason S

There are steppers that are plenty strong. You just need the driver. The nice thing about a hobby servo is that the feed back is built into it. You just need to provide the pulse train. The servo will keep trying to achieve position. ERS

Reply to
Eric R Snow

On Wed, 17 May 2006 21:46:55 +1000, Jason S top-posted:

"stronger"? How about 3.1V 4A, 300 in-oz holding torque? I've got one you can have for ten bucks. ;-)

Cheers! Rich

Reply to
Rich Grise

should be OK as long as you don't overload the motor or the controller.

Bye. Jasen

Reply to
Jasen Betts

Yeh, the servo sounds pretty good actually. However I read somewhere that they spin at full speed when the angle they need to turn is close to max angle, e.g. 160 degrees, but not as fast for shorter distances/angles. Can the speed of these things be controlled? I wouldn't think so.

Jase

Reply to
Jason S

shit, that sounds pretty powerful, haha. thanks for the offer Rich... I might get back to you on that. Haven't decided yet =)

Jase.

Reply to
Jason S

I seriously just might take him up on it if you don't. It DOES sound like a fairly powerful stepper, and that's what I'd like to use for the tilt on the camera pan-/tilt-head I'm trying to develop. (I've already got the pan part ready to go. Thanks, folks, but don't try to sell me a "pre-built" kit - I know that such animals already exist, but I'm doing it from the "scrounge-pile" purely for the sheer "Am I able to make it happen?" of it - call it a self-administered, self-graded "Final Exam" in computers/electronics/robotics/scavenging/MacGuyvering :) )

A couple-few questions for Rich, though -

1) I know an inch-ounce is the force the pivot will see if an ounce of weight (assuming we're at 1 earth gravity) is placed on a 1 inch lever. And a ft-lb is the force of a pound of weight working against a 1 foot lever. So can someone check me here? (300In/Oz) / 192 (12 inches to a foot times 16 ounces to a pound = 192In/Oz per ft/lb?) = about 1.6 ft/lb? Or have I got the conversion hopelessly screwed up?

2) Is this 300 in/oz/1.6 ft/lb measured directly on the motor shaft, or is this after a reduction package, and if reduction, is the geartrain a worm, pancake type, or maybe even a combination of both?

3) How many phases (I'm assuming 4) and control wires? (Assuming 6) and are the phases tagged, or do I have to play "What happens if I energize in this sequence?" to find out?

4) What's the step angle per phase?

5) If it's supposed to be 3.1V@4A, got any suggestions for how I should expect it to behave if I were to try to power it from a 12V@1.5A supply? How about 5V@2.5A? (Besides the most intuitive "All the smoke will probably leak out - whether it leaks out of the PS or the stepper is questionable" scenario - IF there's a viable alternative - right now, those are the flavors of juice I can supply easily. Hmmm... brainstorm just hit: I've got multiples of that PS - Do SMPSes parallel OK? That'd give me 5V@5A that I could mess with regulating down to 3.1V if I absolutely have to.)

6) (And last - I think) What kind of physical dimensions and weight on this unit? Shaft size? Length? Splined, smooth, or geared? Gear or spline pitch/tooth count (if applicable)?

7) (This is really last) How many do you have/want to get rid of?
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Reply to
Don Bruder

Greetings Don, It's nice to be able to post something I actually know something about. I fiddled around with steppers a few years ago and actually built some metalworking stuff with them. If you want to make 'em spin faster the trick is to use higher voltage with resistors to limit the current. So go ahead and drive that 3.1 volt stepper at 12 volts but limit the current to 4 amps. SMPSes don't work well for these because they can destroy your controller. If your controller is robust enough then I suppose you could go that way, but I don't know enough about electronics and stepper driver chips to know for sure. When I tried to use a SMPS the voltage spikes destroyed the driver chip. The driver chip controlled transistors which controlled the stepper. The best power supply I made consisted of a surplus toroidal xmfr, a full wave rectifier, and a cap. I drove 5 volt rated steppers with 40 volts and used big resistors for current limiting. There are more than one type of stepper driving chip available. The ones I used took step and direction inputs and output the switching sequence. It was a cheap device. It did not have an idle current limiting feature though. This is important if you are driving the steppers at high current. The stepper can be driven at high current for speed and torque but when the stepper is idle this same current will overheat the motor. The best system I built was a kit from an outfit called CAMTRONICS. The guy who sold these kits is named Dan Mauch and is a good guy. If he still sells stepper driving kits they will be better than the one I bought from him because he is always trying to improve his products. Cheers, Eric R Snow, P.S. You want to avoid high voltage rated steppers. The highest stepping rates at the highest torque will come from driving the low voltage motors in the manner described above. And finally, it's been a few years since I messed with steppers so I may have given some erroneous info above.

Reply to
Eric R Snow

I built one using the steppers from old dot-matrix printers. My camera is fairly heavy (Sony Mavica MVC-FD7) but it moves smoothly.

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Reply to
Wildepad

My application will be using a 555 to provide a clock signal for a bi-directional counter chip, which will be driving a one-of-four decoder, which will in turn be driving four optocouplers, each claiming to be good for switching up to 70 volts @ 2.5 amps on the output side, to drive the bases of four Darlingtons, each rated for 60 volts, 10 amps continuous/15 amps peak across the C-E junction. In other words, the "logic power" side is going to be completely isolated from the "high power" side of things, electrically speaking, and the high power side is over-engineered to the point of nearly ridiculous.

The logic side should never, under any but the most incredibly unlikely batch of multiple "oops, shouldn't have done that" circumstances happening simultaneously (Any one of which would most likely destroy the whole thing anyway) see anything higher than TTL-logic level voltages.

Considering how "beefy" the high power side of the controller is going to be, and the isolation provided by the optos (the datasheet claims

between the high power and logic level sides, I don't foresee the PS being able to do much damage - Maybe, at worst, blow one or more of the Darlingtons, or in some *TRULY* bizarre combination of conditions nuke the optos, but not much else. Or am I completely naive?

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Reply to
Don Bruder

I'm way too lazy to check your arithmetic here, and I can dismiss the rest of your questions with an "I have no idea". :-)

It'd pretty hefty - maybe five pounds or so; here are a couple of pix:

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The name plate kinda got taken upside down, but I've fixed that. :-)

No Idea - I presume at the shaft. That's where I'd measure it, anyway. :-)

I dunno - it's got nine wires, and I haven't ohmed any of them out - I guess I should... Crap. I can't find my little Micronta toy DVM, and the little VOM would just read continuity - would that help? Yeah, why not - it's another opportunity to go dig around my boxes...

OK, here goes:

W = White, V = Violet, Bk = Black, O = Orange, G = Green, Y = Yellow, R = Red:

The wires are: Wh/V, Wh/O, Bk, O, G/Y, Y, R, W/Y, W/R. (that's body/stripe)

So, to continuity: W/V | W/O | Bk | O | G/Y | Y | R | W/Y | W/R W/V - no yes no->

W/O - yes no->

Bk yes - no->

O yes no->

G/Y no no no no no no no no no Frame- yes

Uh, oh - I see a pattern emerging here!

Y - wh/y R - wh/r W/Y W/R.

So it apparently has four independent coils, which are probably polarized by the stripe in the wires. :-)

It says, "200 steps per rev", so it'd just take some arithmetic, presumably.

12V would burn up the windings, and 1.5A isn't enough to get it to step effectively under any practical load.

I seriously doubt if SMPSs can be paralleled, unless they're designed to be ganged. They'll put each other into voltage limit or something - I hear it's really horrific =:-O .

I'd try to find a supply that can do the 4 amps, and with some voltage compliance, like 6 or 12v, and drive the windings with a constant current sink.

Hopefully, the pix cover this - here's some "actual" dims: shaft dia: .375 Two flats, 90 degrees apart, .340 from flat to opposite side of shaft Mounting flange: 3.40" square, with a round 2.875 projection about .125 tall; Mounting holes on a square, on 2.75" centers, diam. about .220

The flatted end of the shaft is sticking out of the end with the flange, about 1", and there's about 1.25" of round shaft sticking out the other end, where the wires come out.

Just the one - it's starting to sound like I should put this on ebay!

Cheers! Rich

Reply to
Rich Grise

With a hobby servo (like in R/C airplanes), it's really easy to control the speed, up to a point. They get a position command from the receiver (or controller), that's a pulse train - I don't remember the exact figures, but it's fairly liesurely, like 20 ms at a few hertz. The pulse width tells the servo what position to go to *now*, so the speed would be limited by how fast the motor can turn. Of course, you can make it go as slowly as you want, by just commanding it to move a little bit at a time.

The last time I worked with these things, there was also a "motor controller" available, where the PWM signal controlled the _speed_ of an external motor, but they're dangerous - if you lose the signal in an R/C airplane, the servos freeze - if you lose the signal to the motor controller, it freezes _at whatever speed it was driving the motor before_. This is a helluva thing when your robot breaks itself at the demo of your new controller system in front of would-have-been customers. )-;

Cheers! Rich

Reply to
Rich Grise

:)

Yikes! That's definitely a hefty beast!

Well, so would I, but I was trying to find out if it was geared, or direct. Looks to be direct - I don't see much available space for a geartrain in the unit. Nothing inherently WRONG with that concept, just sorta surprising for such a beefy stepper. The "big boys", torque-wise, seem to almost always be a "weakling" coupled to a heavy reduction geartrain of some sort.

Sounds reasonable, with G/Y being a frame-ground.

Good ol' 1.8/step, then. Fairly typical.

I kinda figured that to be likely. Worth asking, though :)

Really? Now doesn't THAT just about suck :( I've got what seems like about half a boatload of switchers laying around, many identical (pulls from old Apple IIes) that are just crying to be paralleled for high-amp work.

Might end up powering the beast off a car battery/solar array charger type setup. Hard to say at this stage. The "development-so-far" has been done with one of the Apple IIe power supplies mentioned.

Yep, pretty well.

Do that, and you just priced me out of the game :( Can't blame ya if you do and it brings in a decent bundle, but at least for right now, I can't "run with the big dogs", money-wise. I'll let Jason have first crack at it, but do drop me a line (See my sig) if things don't work out there and you still want to part with it for cheap-ish.

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Reply to
Don Bruder

haha, OMG, you ARE serious! Half the stuff I don't know what you're even talkin about! =)

Jase

Reply to
Jason S

All but #1 and #7 are fairly important things to know about a stepper's capabilities, with #s 2, 3, 4, and 6 being the most critical for figuring out which one to use in your application.

#2: Torque - 300 inch-pounds of holding torque means that it will keep

300 ounces (A bit less than 19 pounds) of force from moving a lever one inch inch long, assuming the lever pivots on the stepper's shaft. A stepper with only 1 inch-ounce of holding torque isn't very strong at all, and will probably not be worth trying to use for turning anything heavy, or anything that needs tobe held to a tight positioning against some outside force acting on it, since a fart from across the room just might be enough to break its step. A "weak" stepper can, in effect, be made "stronger" by having it drive a reduction geartrain, so that the 1 inch-ounce unit driving a 16:1 reduction could hold position against a pound of force being applied to that one inch lever, instead of an ounce. The tradeoff is, of course, speed - It's now going to take 16 turns of the stepper shaft to get one turn of the output shaft of the geartrain, or 16 steps of the stepper to get one step worth of rotation at the output shaft of the reduction package. Worm-and-wheel, and "pancake" style gears are two construction methods for reduction units - Each has its plusses and minuses that are *WAY* off topic for an electronics newsgroup, and beyond my abilities to explain, besides.

#3: Phases/control wires - Steppers come in various "flavors" - 2 and 4 phase being most common, though others exist. Phase count influences #4 (step angle) #2 (holding torque) and wiring - A two phase stepper probably has either 3 or 4 wires to connect, and wil have a difference stepping sequence than a 4 phase unit. 4 phase probably has 6 or 8 wires that need to be connected, and in both cases, one needs to know the stepping sequence to make the stepper do anything other than sit there and twitch spastically. Energizing one phase of a stepper results in it taking exactly one step. Energizing the next phase in the step sequence makes it take another step, etc, so to get it to turn, rather than wiggle, you repeatedly energize the phases in the step sequence. Figuring out the step sequence from a handful of wires and the info on the stepper's nameplate can be... "interesting". "So I need to step it as phase1, phase3, phase2, phase4 to get it to turn clockwise - OK, which pair of wires is phase1? Repeat for phases 2-4. Now did I get the polarity right? (IE, I've figured out that Black and Red are phase1, but if I put the juice to it as red(+) and black(-), it steps clockwise, and reversing the polarity makes it step counterclockwise)

#4: Step angle: How far does the shaft turn for each step that happens?

1.8 degrees per step is very common, but other angles are possible - I've encountered 180 degrees per step units, 3 degrees per step, etc - *LOTS* of possibilities. #3 (Phases) usually has a big impact. Smaller step angles usually mean higher holding torque (but not always) and more phases usually mean smaller step angles (but again, not always) Smaller step angles ALWAYS mean more positioning precision is possible.

#5: Power requirements - Too much, and all the magic smoke leaks out of something (maybe the power supply, maybe the stepper, maybe parts in between) Too little, and the stepper doesn't do much of anything. There's a "happy medium" to be found for each one. The one Rich is talking about wants 3.1 volts at 4 amps. Volts and amps can be "fiddled with" in various ways to make it possible to apply more amps and less volts, or more volts and less amps, or some combination, to let you feed the beast with what you've got. But there are limits before the smoke leaks out.

#6: Physical properties - "Will it fit where I want to put it? And if it will, can I get it mechanically connected to what I want it to drive?" It's more than a bit difficult to mate a round-with-a-flat shaft to a driven gear, ferinstance, and trying to fit a smooth round shaft to a splined input is probably going to work about as well as a lead balooon.

Hope this helps... :)

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Reply to
Don Bruder

The newer digital servos don't have the same limitations[1] as older analogue designs. They provide full torque for minimal displacements, programmable endpoints and probably other features I don't know about. I don't know if speed is controllable beyond slowly ramping the input. Search the web for 'digital servo' and you'll find Futaba, HiTec and others have a range. They aren't as cheap as regular servos, but they are pretty good.

Tim

[1] It's a limitation in normal servo operation, but when modified for continuous rotation that "limitation" facilitates (fairly crude) speed control using the original electronics.
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Reply to
Tim Auton

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