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'Stepper Motor Driver using PIC'
1997\01\03@201959 by rgreason

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I have acquired three types of used stepper motors which I should like to
try and drive using a PIC,  and wonder if I can use any of them on my most
recent project.  There is an outside chance that they may be broken ,  but
perhaps I can test them somehow.

They are 7.5deg/step except one which appears to have an optical disc on
one end with associated circuitry, the pattern on which is just
radial lines like a vernier perhaps.  They all have six wires and the same
kind of connector (female - inline - six pole).  The one with the optical
disc has a further two wires on a second female connector,  which I think
must be power.

I think that they must all be permanent magnet types except the optical
disc one.

Does anyone know...Is there a way to test these items using a PIC
perhaps?  Or should I just get a driver circuit from Maplin?  The
former option is the most interesting,  and the latter,  the most
expensive.

BTW does anyone know what is the finest of incremental angle that a
stepper motor can produce?

Regards - Roy...

------------------------------
Roy Greason....
http://www.colours.demon.co.uk
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1997\01\04@093955 by Fred Thompson

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Reply mixed into original message.

Fred Thompson
spam_OUTfthompsoTakeThisOuTspammail.win.org


On Fri, 3 Jan 1997, Roy Greason wrote:

> I have acquired three types of used stepper motors which I should like to
> try and drive using a PIC,  and wonder if I can use any of them on my most
> recent project.  There is an outside chance that they may be broken ,  but
> perhaps I can test them somehow.
>

       First step is with an ohm meter.  This tells you which wires
connect to the coils.  Next you should energize these coils to find the
order that makes the motor continue to turn in one direction.  Generally
you can think of stepper wiring in two ways.  5 wire and 4 wire.  5 wire
designs have a common wire which is connected to V+ and the other four
wires are brought to ground in sequence to turn the motor.  4 wires
energize the coil pairs alternately reversing the polarity on the two
wires every other step.  6 wire motors are like the 5 wire case with two
wires which are tied to V+ as two sets of coils are separated.  I think
you can also treat these as 4 wire motors if that is more useful.  Its a
real education to sit down with a power supply and some alligator clips
and step these around a few times.

{Quote hidden}

       Once you have the order to energize the coils, you can write PIC
code to drive 4 pins to step the motor around.  You will need some drive
transistors or H Bridge chips to get the current up.  Since your
tinkering with these (i.e don't have spec. sheets), be sure to have some
method for varying the speed.  Steppers will go as slow as you want, but
their top speed will depend on several things (shaft load, stability of step
frequency, stability of supply and current amp,...).  So allow some way
to vary the speed, some PICs have analog inputs on which you may hang a pot.


> BTW does anyone know what is the finest of incremental angle that a
> stepper motor can produce?
       I have some that single step at 400 steps per circle.  They were
surplus and I haven't seen them since.  You can also "micro step"
Steppers.  This is done by slowly changing the current flowing through two
adjacent step phases (or PWM their lines).  The idea is that the motor
will slowly move from one step to the next.  I don't think there is a clear
or consistent relationship between the applied currents and the position
of the motor so some fine resolution feedback will be needed.
       You might want to consider the book "Direct Drive Robots, Theory
and Practice" by H. Asada, K Youcef-Toumi which should be available at
your local library through interlibrary loan.  This book contains
excellent descriptions of Steppers and variable reluctance motors (my
guess is that the one without the permanent magnet is a variable reluctance).

>
> Regards - Roy...
>
> ------------------------------
> Roy Greason....
> http://www.colours.demon.co.uk
> ------------------------------
>

Hope this helps
Fred

1997\01\04@151850 by Brent Miller

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At 09:17 PM 1/3/97 +0000, Roy Greason wrote:
[Text deleted]..
>BTW does anyone know what is the finest of incremental angle that a
>stepper motor can produce?
>

I recall having a catalog of rather unusual stepper motors a number of
years back (OK - a large number!).  These were based on a reducing gear
called a "harmonic drive", and had something like 1500 or 2000 steps
per revolution.  They were made by United Shoe Machinery (USM).  I
haven't heard much of either the motors or the company recently, and
a quick web search didn't find anything.

The harmonic drive is a high ratio gear mechanism consisting of 3
pieces.  Picture a fixed hollow cylinder with gear teeth on the inside.
Assume there are 501 teeth on the inside of this cylinder.

Now place a slightly smaller very thin walled hollow cylinder inside
the fixed cylinder and put 500 gear teeth (same pitch as the fixed
cylinder) on the outside of this thin and flexible cylinder.  Imagine
that it is a tin can with teeth on the outside.

Finally, but a rotor inside the can.  The rotor has a single ball
bearing that presses against the inside of the can at one point,
causing the can's teeth to engage the teeth inside the fixed cylinder
at the point of contact.  Now rotate the inner rotor one revolution,
and the can will have turned 1/500 of a revolution because there is
one more tooth on the fixed cylinder than on the can. The tooth on the
can that is engaged with the fixed cylinder is the one next to the
one that was engaged originally.

To make a stepper out of this, remove the inner rotor.  Make the can
from magnetic material, and place a set of coils outside the fixed
cylinder so that the can is pulled into contact by one of the coils.
Rotate the field one turn by energizing coils in sequence, and you
get 1/500 of a revolution on the can.

I hope this picture is clear enough.  Does anyone know if these
contraptions are still available?

Brent W. Miller
Menlo Park, CA
.....bmillerKILLspamspam@spam@scruznet.com

1997\01\04@154141 by Mike

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{Quote hidden}

Your description is great but,

"A picture paints a thousand words but why can't I understand such a clear
and concise description.."

Anybody know where theres a picture of this 'device' ?

Rgds


Mike

Socrates once gave the advice to "by all means get married... If you
get a good wife you will become happy, if you get a bad one you will
become a philosopher."

1997\01\04@191741 by Walter Banks

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Brent Miller wrote:
>
> >BTW does anyone know what is the finest of incremental angle that a
> >stepper motor can produce?

> These were based on a reducing gear called a "harmonic drive", and
> had something like 1500 or 2000 steps per revolution.  They were
> made by United Shoe Machinery (USM).  I haven't heard much of
> either the motors or the company recently, and a quick web search
> didn't find anything.

Another proof there just isn't a free lunch. Good high ratio gearing
but a poor single tooth sure had to take a lot of torque. There were
a lot of variations on this system including some whose flexible case
deformed to add to the amount of gear surface area engaged.

Walter Banks

1997\01\05@015134 by Matthew Mucker

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>At 09:17 PM 1/3/97 +0000, Roy Greason wrote:
>[Text deleted]..
>>BTW does anyone know what is the finest of incremental angle that a
>>stepper motor can produce?
>>

Actually, by varying the current (or voltage?) in each of the windings, you
can make as many steps per revolution as you want.  Check back issues of my
favorite magazine, Circuit Cellar Ink, for details.  There are several
articles in recent years about stepper motors.

-Matt

1997\01\05@095637 by Martin Nilsson

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Since I know many of the PIC list members are interested in
mechatronics, I think some comment on Harmonic Drives (HD) is
warranted here. I'll keep it short.

The HD is an exceptional device. Its most notable feature is an
extreme torque/weight ratio. A 100 gram HD can take over 10Nm
continuous torque! Stiffness high, backlash very small. I know of no
other device with comparable performance. HDs are key components in
light-weight, high-torque devices, such as robots.

A common gear ratio for an HD is 1:100. If you multiply this by your
stepper resultion (eg 1:180), you have the total resolution. 1:18000.
Efficiency: About 50-70%. Difficulty: Mounting must be extremely precise,
or you risk damaging the HD. Shockloading should be avoided. Cost for
the smallest types: In the range 200-1000 USD. Lead times: Typically a
few months.

Walter Banks wrote:

{Quote hidden}

The reason why HDs can take so much torque is exactly because the
number of teeth in contact is quite large! This is particularly true
for the relatively recent S-shaped teeth. I would rather believe the
reason the combination isn't available is that it is much easier to
put the HD directly on the shaft of (or around, since HDs can be made
hollow) a suitable stepper motor. You will not earn very much by
combining the two in one unit.

-- Martin Nilsson

(PS: I'm not paid by any of the HD companies :-)

Martin Nilsson                           http://www.sics.se/~mn/
Swedish Institute of Computer Science    E-mail: mnspamKILLspamsics.se
Box 1263, S-164 28 Kista                 Fax: +46-8-751-7230
Sweden                                   Tel: +46-8-752-1574

1997\01\06@100459 by Kevin Krauel

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----------

>>To make a stepper out of this, remove the inner rotor.  Make the can
>>from magnetic material, and place a set of coils outside the fixed
>>cylinder so that the can is pulled into contact by one of the coils.
>>Rotate the field one turn by energizing coils in sequence, and you
>>get 1/500 of a revolution on the can.
>>
>>I hope this picture is clear enough.  Does anyone know if these
>>contraptions are still available?
>
>Your description is great but,
>
>"A picture paints a thousand words but why can't I understand such a clear
>and concise description.."
>
>Anybody know where theres a picture of this 'device' ?

----------

I am not familiar with harmonic drives with integrated stepper motors (as
described in the previous message), but I can suggest a web site with a
description (including pictures) of a harmonic geardrive.  Please check out
the following web page:

       http://hdsystemsinc.com/techpage/operatn.htm

-Kevin.

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