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'[EE]: 12 Vdc motor advice?'
2001\10\16@122917 by Michael Vinson

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Greetings all. My kids have been begging me to build some kind of
robot. I'm thinking of something like an 18" square platform (wood,
initially), cruising around via radio control from my main computer
setup. It'll have an ultrasonic ranger (that I've already built) on
it, a PIC to talk (via RF, I've already got that part done) to the
computer, etc. The place where I'm stuck is the motors. I'm thinking
of two drive wheels, each with its own PWM-controlled motor with a
sensor to measure how fast and how far each one has rotated. So what
I need are two fairly hefty, 12-Vdc motors, reversible. I have an old
windshield wiper motor from a former car, but it seems (?) to have
the field in series with the armature (?)... at any rate, I can't
get it to go in reverse, which is crucial since if I use two matching
motors, one will have to go one way (CW, say), while the other goes
the other way (CCW). It doesn't need to go very fast...60 RPM or so
would be fine.

Any advice? Pointers to advice? I suppose it should go without saying
that I don't want to spend much money on these things, so any hints
on finding them used or otherwise cheap would be much appreciated
(the reason I thought of windshield wiper motors is that I figured
I could get them from a junkyard).

Thank in advance!

Michael

Thank you for reading my little posting.


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2001\10\16@150204 by steve

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> Any advice? Pointers to advice? I suppose it should go without saying
> that I don't want to spend much money on these things, so any hints on
> finding them used or otherwise cheap would be much appreciated (the
> reason I thought of windshield wiper motors is that I figured I could
> get them from a junkyard).

Electric window motors are about the same size and torque and
will run in either direction (just swap the two wires over).
Be warned - like anything automotive, they suck lots of current
(30A startup).
I don't know about robot platforms (they are intended for a low duty-
cycle application), but I've just used one for shifting an automatic
transmission and it is quite well behaved. ie. Not too noisy, start-up
and run-down times fairly constant, speed and torque quite
reasonable, etc.

Steve.


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Steve Baldwin                Electronic Product Design
TLA Microsystems Ltd         Microcontroller Specialists
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Auckland, New Zealand        ph  +64 9 820-2221
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2001\10\16@155057 by Allen Mahurin

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As Steve said, with window motors, there is one thing
to be wary.  They were designed for intermittent
operation.  It doesn't take long to put a window up or
down.  I know guys that have complained about burning
up their window motors when driving 'bot platforms
because of this.  I think most of them have gone with
wiper motors, and I haven't heard any complaints.
Maybe some can reverse direction and others can't?

Good luck,

ATM

--- Steve Baldwin <.....steveKILLspamspam@spam@TLA.CO.NZ> wrote:
{Quote hidden}

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2001\10\16@182302 by Thomas McGahee

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The only way you can make the series-wound motor reverse is
to open up the motor, and re-wire the interconnect wiring
to be in parallel instead of series. This effectively
makes the motor run at half the voltage and double the current.

Then you can use a DPDT relay to provide the crossover
switching to make it go forward or reverse. You can also use
power fets in an H bridge to do the same thing... depends on
what kind of junk you have hanging around...

Fr. Thomas McGahee

{Original Message removed}

2001\10\16@201141 by Gennette, Bruce

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Most of the robotics guys would say to 'find' some old printers and salvage
the motors from them.  Most large companies (and often their suppliers) find
it un-economic to repair cheap bubble printers so they just throw them out.
Ask if you can look in the dumpster out the back of one of them and most
likely they'll set some aside for you so you don't have to climb in.

Anyway you'll end up with a collection of 5 and 12 volt stepper motors which
give accurate control and built-in measurement of distance covered (less
slipage and radial loses in turns).  You'll also have some sets of gear
trains and assorted cogs, drive belts, rigid chassis, rubber feet, etc and
high power driver transistors, fancy shaped LEDs, etc, etc.  Steppers are
simple to control with a pic.

Bye.

{Original Message removed}

2001\10\16@213414 by Barry Gershenfeld

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>The only way you can make the series-wound motor reverse is
>to open up the motor, and re-wire the interconnect wiring
>to be in parallel instead of series. This effectively
>makes the motor run at half the voltage and double the current.
>
>Then you can use a DPDT relay to provide the crossover
>switching to make it go forward or reverse. You can also use
>power fets in an H bridge to do the same thing... depends on
>what kind of junk you have hanging around...
>
>Fr. Thomas McGahee

Couldn't you open it up and reverse just the field winding?

That would make it run the other way, not reversible, but
you could bring out the field wiring and use your relays or
H-bridge to reverse it.

The interesting feature of a series-wound motor is that
at low speeds it draws a lot of current and gives
you a lot of torque.

Barry

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2001\10\17@040232 by Roman Black

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Michael Vinson wrote:
>
> Greetings all. My kids have been begging me to build some kind of
> robot. I'm thinking of something like an 18" square platform (wood,
> initially), cruising around via radio control from my main computer
> setup. It'll have an ultrasonic ranger (that I've already built) on
> it, a PIC to talk (via RF, I've already got that part done) to the
> computer, etc.

Sounds nice!

{Quote hidden}

The motor you have is a DC "series motor". They have
massive low speed (stall) torques and are typical
in old Fords etc from the 60s and 70s. Now most wiper
motors are smaller cheaper permanent magnet types.

You can only reverse a series motor by reversing
the field OR the armature. :o)
You will need to get 2 matched motors. Then to make
your robot go straight you may still need to add
optical disks or encoders to the motors.

A great option is to find an old HEAVY fax machine,
try begging around the business equipment shops
for an ex-trade in or old dead fax. The old ones
have 2 matched LARGE steppers, and some gears.
This will give you precise positioning and adds
a really nice sound and style of movement. Stepper
bots move like little industrial robots, where
DC motor driven bots always move a bit like
remote controlled toys, to my eye. :o)
-Roman

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2001\10\17@041914 by Roman Black

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Gennette, Bruce wrote:
>
> Most of the robotics guys would say to 'find' some old printers and salvage
> the motors from them.  Most large companies (and often their suppliers) find
> it un-economic to repair cheap bubble printers so they just throw them out.
> Ask if you can look in the dumpster out the back of one of them and most
> likely they'll set some aside for you so you don't have to climb in.
>
> Anyway you'll end up with a collection of 5 and 12 volt stepper motors which
> give accurate control and built-in measurement of distance covered (less
> slipage and radial loses in turns).  You'll also have some sets of gear
> trains and assorted cogs, drive belts, rigid chassis, rubber feet, etc and
> high power driver transistors, fancy shaped LEDs, etc, etc.  Steppers are
> simple to control with a pic.

Agreed! Most of the office equipment shops
do aggressive marketing with trade in deals
and finance, and they end up with a whole
storeroom of old faxes and copiers they have
to pay someone to dispose of. A nice phone
call usually nets you some good stuff.
-Roman

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2001\10\17@071937 by code

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hello,
   i'm sorry but this is abit of side tracking. I'm a newbie in
electronics. You'd mentioned RF in your post to the piclist and I'm very
interested in knowing how to build one. Is it possible to send the
schematics to me?

Regards,
Tan

{Original Message removed}

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