Stepper Motors

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Lenp
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Stepper Motors

Post by Lenp » Sat Oct 20, 2012 8:27 am

Hi All....

I have a pending design for a stepper motor application. It needs to turn a certain (variable) number of degrees with a single input trigger.
I do not expect a problem with the programming of the stepper control and I know there are dedicated stepper driver chips, but the question is...
Should I go with a unipolar or bipolar stepper. The stepper will not need a lot of high torque.

Opinions!

Thanks,
Len
Len

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Bob Scott
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Re: Stepper Motors

Post by Bob Scott » Sat Oct 20, 2012 11:52 am

The unipolar, I think that's the one with the center-taps, is the most versatile. You can use it both ways.
eg: a unipolar motor with both windings tapped 6V-0V-6v can be used as a bipolar 12V.

It dissipates more power as a bipolar this way but I have not heard any negative comments about the extra power. Anyone?
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Lenp
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Re: Stepper Motors

Post by Lenp » Sun Oct 21, 2012 6:07 am

Thanks Bob,

So with the unipolar I can run it in either mode and that looks like the best choice! I'll have to scour the junk box and see what I have.
I just modified an old 12 position rotary switch by taking out the stop and the detent then wiring the contacts 123412341234.
I can use it to manually run the stepper to determine the connection configuration.
Thanks.
Len
Len

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kheston
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Re: Stepper Motors

Post by kheston » Mon Oct 22, 2012 9:00 pm

LenP,

When you've chosen a driver, please post which one. I've got a few steppers laying around that are aching for a project.

--K
Kurt - SF Bay

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dacflyer
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Re: Stepper Motors

Post by dacflyer » Tue Oct 23, 2012 4:34 am

ya know i bet a lot of good drivers could be found in old copy machines or scanners and such...
them machines are full of stepper motors.

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Bob Scott
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Re: Stepper Motors

Post by Bob Scott » Wed Oct 24, 2012 9:16 am

Lenp wrote:Thanks Bob,

So with the unipolar I can run it in either mode and that looks like the best choice! I'll have to scour the junk box and see what I have.
I just modified an old 12 position rotary switch by taking out the stop and the detent then wiring the contacts 123412341234.
I can use it to manually run the stepper to determine the connection configuration.
Thanks.
Len
If you're anywhere Anchor Electronics in San Jose, they used to have 12 position 4 pole enclosed rotary switches in their store that were not in their advertised price list. I've used those before as a bipolar stepper motor selector switch.
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Lenp
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Re: Stepper Motors

Post by Lenp » Wed Oct 24, 2012 9:41 am

Thanks Bob,
I'm 2,800 miles away so as close as I can get is their web site and I didn't see any steppers or switches. I have several and it is likely one will do this job.

Thanks,

Len
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jwax
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Re: Stepper Motors

Post by jwax » Fri Oct 26, 2012 10:24 am

Sparkfun has a small variety of stepper motors and drivers, along with an "Application forum" of users swapping thoughts on the products.
https://www.sparkfun.com/categories/31
Nice folks to deal with too!

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Re: Stepper Motors

Post by jwax » Mon Nov 05, 2012 10:53 am

Question on steppers: Is it possible to change the "holding current" independent of the "running current"?
I have a current control on the driver, but when I crank it up to get good running current (turning torque), the holding current goes up too. This wastes battery power and is unnecessary. Heats up the motor too!
Is this typical of drivers, or is it a fact of life for all steppers?
In other words, I need turning torque, and very little holding torque. Is that possible?
John

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Bob Scott
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Re: Stepper Motors

Post by Bob Scott » Mon Nov 05, 2012 11:22 am

jwax wrote:Question on steppers: Is it possible to change the "holding current" independent of the "running current"?
Sure. When actively stepping the circuit becomes inactive, then lower the power supply voltage going to the windings. How you incorporate that would be dependent on the circuit you are using.

Ok, ok ,so a microprocessor could do it easily. :yuck:
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Re: Stepper Motors

Post by jwax » Mon Nov 05, 2012 1:15 pm

Thanks Bob! I'm using this driver:
https://www.sparkfun.com/products/10735

Not knowing much programming language, are you saying that I can tell my Arduino (which is talking to the driver) to reduce current to the motor during "off" time? Could you steer me to the code that would do that?

Appreciate the help!

John

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Lenp
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Re: Stepper Motors

Post by Lenp » Mon Nov 05, 2012 1:19 pm

Stepper holding...

As an alternative to reducing the power supply and the holding voltage/current I think the average power would drop if after the last step in a sequence, you change that same phase to either a very fast frequency or a PWM output. A micro could easily do that and it may save a pin and the cumbersome need to switch the power supply.

Comments!

Len
Len

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Bob Scott
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Re: Stepper Motors

Post by Bob Scott » Mon Nov 05, 2012 5:21 pm

Lenp wrote:As an alternative to reducing the power supply and the holding voltage/current I think the average power would drop if after the last step in a sequence, you change that same phase to either a very fast frequency or a PWM output.
That is a brilliant idea. It is the same way that RC electric motor controllers increased efficiency. Old controllers used 50 Hz. Then the newer high efficiency ones used 3-4 KHz.

Raise the frequency across the winding to 3-4 KHz minimum so that the windings' inductance prevents high frequency current. If you choose 50% duty cycle, then it works the same as a class D amp, the net DC voltage will be 50% and your winding current will drop to half.

I was actually pondering the same idea for driving stepper motors with pulse width modulated high frequency "simulated" sine waves to prevent the jerkiness. Imagine a smooth quiet stepper motor!

NOte: I am not familiar with programming arduinos. The last microcontroller I was attempting to program with machine code was a 6809 micro. That was a long time ago ~1983, before IBM started making PCs. I made up my own tables to convert machine code to hex. I was programming "down to bare metal". Writing the machine code on a my Commodore PET with 8K kept crashing before I could save it. BAAAD experience. Also I don't like associating with cliques of geeks who talk in a code of acronyms, so I disassociate myself from that business.
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Lenp
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Re: Stepper Motors

Post by Lenp » Tue Nov 06, 2012 5:53 am

Thanks Bob,
I have worked with VFD's and am impressed with the performance difference between them a 60hz standard motor. The controllers offer many features, smooth operation with a somwhat attractive whine, ramped start and stop, E-stop (by locking the windings), current, torque, time or speed limiting, remote control, synching...the list just goes on.

For some reason I am still involved in Basic programming, and never got onto the arduino system. I suppose it all started a lifetime ago with learning Quickbasic, migrating to the Parallax Stamp and now to the Picaxe, an enhanced Pic Chip. So now I am into using the PicAxe line of processors, which have a very familiar and simple dialect, a rich command set that includes PWM, plus free programming software with a simulator. They offer a wide range of processors that are almost as cheap as the naked Pic chip, and the whole system is supported by a great forum with the manufacturer's tech support involvement.

Granted the code is not as compact, flexible, or as fast as other languages, but, despite some wrinkled noses from the code junkies, what really counts is that it works for me!

Len
Len

“To invent, you need a good imagination and a big pile of junk.” (T. Edison)
"I must be on the way to success since I already have the junk". (Me)

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