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'Need gear motors for PIC 'bot'
1998\07\13@235250 by Dan Larson

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Can anyone please point me to some gear motors so that I can
start my PIC 'bot project?

I am looking for 12vdc 60-120 rpm smallish DC gearhead motors
for My PIC bot.  I will be using a 12v 1.2ah lead acid battery
for the power source.  The platform will be small because it
will be my first.  Something about the size of a floppy
stepper would be about the right size.  About $10 would
be the right price.

I am having a world of trouble finding good links for
anyone selling DC gearhead motors.

How powerful are modified hobby servo's?  Is this a agood way to
go for a small rolling platform?

Your pointers will be greatly appreciated!

Dan

1998\07\14@001530 by Timothy D. Gray

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Hobby servos can be quite powerful, the HD type can overpower a human
hand.(Hurts too!)

On Mon, 13 Jul 1998, Dan Larson wrote:

{Quote hidden}

1998\07\14@174923 by Sean Breheny

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On Tue, 14 Jul 1998, Timothy D. Gray wrote:

> Hobby servos can be quite powerful, the HD type can overpower a human
> hand.(Hurts too!)

Yes, but they are not intended to be continuously rolling. They go to a
certain angular position depending on the PPM that they receive on the
signal line.


Instead, I would reccomend taking a look at edmund scientific, Dan. They
have quite a variety of geared motors. I have never bough one from them,
though.

A servo would be a good item to move a steering mechanism, but you could
also just have the motors on one side spn faster than the other side.

Sean


{Quote hidden}

1998\07\14@182659 by Lee Jones

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>>> Can anyone please point me to some gear motors so that I can
>>> start my PIC 'bot project?

>> Hobby servos can be quite powerful, the HD type can overpower a human
>> hand.(Hurts too!)

> Yes, but they are not intended to be continuously rolling. They
> go to a certain angular position depending on the PPM that they
> receive on the signal line.

With the standard control circuit board installed, that's true.
(Actually, the position depends on pulse width, nominally 1.0
to 2.0 milliseconds for end to end travel; repetition is 20-50
pulses per second.  I assume PPM was a typo for PWM.)

But there's no reason why you can't take a servo, ditch the
factory control board, bring out the motor leads (using the
existing wires), and run it as a geared motor.  Think of it
as a kit of motor, gears, bearings, output shaft, and case.
Even if you have the gears, getting all the shafts with just
the right spacing so that the gears mesh with the right
clearance is time consuming.

You'd have to create your own feadback mechanism though.


Tamiya also makes a couple kits of motor, gears, and frame.
You assemble them with various pieces to get different ratios.
They have both a spur gear set and a planetary gear set (with
higher reduction ratio).
                                               Lee Jones

1998\07\14@185614 by Sean Breheny
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On Tue, 14 Jul 1998, Lee Jones wrote:

> > Yes, but they are not intended to be continuously rolling. They
> > go to a certain angular position depending on the PPM that they
> > receive on the signal line.
>
> With the standard control circuit board installed, that's true.
> (Actually, the position depends on pulse width, nominally 1.0
> to 2.0 milliseconds for end to end travel; repetition is 20-50
> pulses per second.  I assume PPM was a typo for PWM.)
>

As I recall, it is actually the gap between pulses that determines
position, and the pulses have constant width, hence Pulse Position
Modulation. I could be wrong. I thought that PPM was used instead of PWM
because it was easier to multiplex PPM to get the multiple channels
needed for RC.

> But there's no reason why you can't take a servo, ditch the
> factory control board, bring out the motor leads (using the
> existing wires), and run it as a geared motor.  Think of it
> as a kit of motor, gears, bearings, output shaft, and case.
> Even if you have the gears, getting all the shafts with just
> the right spacing so that the gears mesh with the right
> clearance is time consuming.
>

I see your point. However, you would still need some type of hardware to
connect the servo to the wheels. This could end up being just as
difficult as just connecting some other type of motor to the wheels. It
really depends on what one finds. The gear on the end of the servo (to
which the control horn attaches) is very small.

> You'd have to create your own feadback mechanism though.
>
>
> Tamiya also makes a couple kits of motor, gears, and frame.
> You assemble them with various pieces to get different ratios.
> They have both a spur gear set and a planetary gear set (with
> higher reduction ratio).

This, would of course, be the easiest to use, I think. But, then again,
cost is a big issue here.

>                                                 Lee Jones
>

1998\07\14@195824 by Philip Starbuck

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<SNIP>

>> Yes, but they are not intended to be continuously rolling. They
>> go to a certain angular position depending on the PPM that they
>> receive on the signal line.

The above comment raises another question, is is there a duty cycle limitation placed on the modified servo under a given load. In normal operation, the servo is commanded to a position then it uses a small amount of current to hold that position.  When used as a drive motor the servo is commanded to turn on and drive in that direction for an extended time, this would seem to cause greater power dissipation in the motor and control board than it was originally designed for.  Are there additional mods required to reduce heating in the motor and/or drive board when a hobby servo is used in this manner?


<SNIP>

cheers,

Philip Starbuck

Philip Starbuck
(909) 792-7917

"There are three principal ways to lose money.  Wine, women, and engineers.  While the first two are more pleasant the third is by far the more certain."
                                               -- Baron Rothschild
                                                       ca. 1860

1998\07\14@212850 by Mike Keitz

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On Tue, 14 Jul 1998 18:54:59 -0400 Sean Breheny <spam_OUTshb7TakeThisOuTspamCORNELL.EDU>
writes:
>On Tue, 14 Jul 1998, Lee Jones wrote:

>
>As I recall, it is actually the gap between pulses that determines
>position, and the pulses have constant width, hence Pulse Position
>Modulation.

The signal over the air is that way.  After processing by the receiver,
the signal applied to each servo is a single pulse of varying width
(duration) corresponding to the width of the gap between two particular
radio pulses.  The absolute phase of these pulses varies somewhat
according to the setting of the other controls, but servos don't notice
that.

I could be wrong. I thought that PPM was used instead of
>PWM
>because it was easier to multiplex PPM to get the multiple channels
>needed for RC.

I think it is done that way so the radio hardware can be optimized to
transmit and receive pulses of constant width.  In an "AM" system the
pulses are RF off, and the spaces are RF on.  Using short constant pulses
keeps the transmitter on almost all of the time, which makes the receiver
less sensitive to noise.

>
>> But there's no reason why you can't take a servo, ditch the
>> factory control board, bring out the motor leads (using the
>> existing wires), and run it as a geared motor.

Almost always there are stops that keep the output shaft from going all
the way around (to protect the potentiometer, which you would remove
anyway).  Often the stops can be removed.  But in some cases the final
gear is just a sector, not a complete circle.  Obviously that won't work.


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1998\07\14@215513 by Mark Willis

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See http://www.seattlerobotics.org/ for links to modifying Futaba
S-148 servos for continuous rotation, at
http://www.seattlerobotics.org/guide/servohack.html <G>

 The SRS members use these servos quite a bit for Robotics.  Now if
only I could make it to more meetings...  (I work weekends.  Drat!)

 Mark

Philip Starbuck wrote:
>
> <SNIP>
>
> >> Yes, but they are not intended to be continuously rolling. They
> >> go to a certain angular position depending on the PPM that they
> >> receive on the signal line.
>
> The above comment raises another question, is is there a duty cycle limitation
placed on the modified servo under a given load. In normal operation, the servo
is commanded to a position then it uses a small amount of current to hold that
position.  When used as a drive motor the servo is commanded to turn on and driv
e in that direction for an extended time, this would seem to cause greater power
dissipation in the motor and control board than it was originally designed for.
 Are there additional mods required to reduce heating in the motor and/or drive
board when a hobby servo is used in this manner?
{Quote hidden}

hile the first two are more pleasant the third is by far the more certain."
>                                                 -- Baron Rothschild
>                                                         ca. 1860

1998\07\14@223148 by Dan Larson

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Thanks to all for the overwhelming repsonse to my request!

I'll keep everyone updated with my progress from time to time, but
don't expect anything soon.  The bot building gets squeezed into
my life last of all, behind wife, kids, job, cigars,.....

Till later....

Dan

1998\07\14@223601 by kotay

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I accidently sent this just to the original poster the first
time, but I think that some people on the list might be
interested...

> I am looking for 12vdc 60-120 rpm smallish DC gearhead motors
> for My PIC bot.  I will be using a 12v 1.2ah lead acid battery
> for the power source.  The platform will be small because it
> will be my first.  Something about the size of a floppy
> stepper would be about the right size.  About $10 would
> be the right price.

> I am having a world of trouble finding good links for
> anyone selling DC gearhead motors.

Jameco (http://www.jameco.com) has a nice 12V gear motor with a
69 rpm no load speed.  Torque is 2.2 kg-cm, max current is
275 mA (max efficiency).  It is 1.5" in diameter and 1.5"
long.  The price is $20 and the part number is 151440.  I
purchased one of these motors and it works well.  It has a
large flatted shaft which makes it easy to connect to
output devices.

Keith

Keith D. Kotay
Dartmouth Robotics Lab
.....kotayKILLspamspam@spam@cs.dartmouth.edu
http://www.cs.dartmouth.edu/~kotay

1998\07\15@121835 by Peter L. Peres

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On Tue, 14 Jul 1998, Philip Starbuck wrote:

> <SNIP>
>
> >> Yes, but they are not intended to be continuously rolling. They
> >> go to a certain angular position depending on the PPM that they
> >> receive on the signal line.
>
> The above comment raises another question, is is there a duty cycle
> limitation placed on the modified servo under a given load. In normal
> operation, the servo is commanded to a position then it uses a small
> amount of current to hold that position.  When used as a drive motor the
> servo is commanded to turn on and drive in that direction for an
> extended time, this would seem to cause greater power dissipation in the
> motor and control board than it was originally designed for.  Are there
> additional mods required to reduce heating in the motor and/or drive
> board when a hobby servo is used in this manner?
>
>
> <SNIP>

In my experience, the motors are rated at the nominal Vcc used in servos
(4/4.8 or 7.2/6Vdc), but the electronics lack the required heatsinking to
supply the stall current for the motor for more than about 5 minutes.

As a rule of thumb, it is relatively safe to run a small DC motor rated
for continuous use under load conditions such, that it takes a current of
1/2 or its stall current at its nominal Vcc, unless otherwise specified.

The electronics used in usual commercial servos, never steer the motor in
proportiaonal mode, they use a chopper (rare) and 'digital' 'H' output
stages (go/no go). So, even a relatively small driver can run a 1.5 Amp
motor as it disspiates only on its switching transistor's Ron's, about
0.25 to 2 Watts.  Reliability however, is another matter.  And remember to
use a FUSE for each motor, especially if your power supply is rated for
more than 4 servos, or is a NiCd or Pb battery, unless you'd like the
wiring to double as smoke bomb/smouldering fire. For usual servos, 1 Amp
each should be OK.

hope this helps,

       Peter

1998\07\15@163954 by Thomas McGahee

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----------
{Quote hidden}

The pulse width is what controls the servo. The specification is
designed to allow a single transmitter to transmit information to
several different servos. This is done by sending out a 'frame' that
is captured by the single receiver and reconstructed into the
individual pulse widths. I will illustrate this with a dumb ascii
drawing of two servo signals being framed.

11100000 signal A
11000000 signal B
1110000011000000 signals A and B combined (2 per frame)

continuous signal looks like this:
1110000011000000111000001100000011100000110000001110000011000000

Normally they do not frame more than 8 servo signals

Try SERVO SYSTEMS CO. for a great collection of geared motors and also
true motors with servo control feedback in the form of
built-in tachometers and also quadrature detectors. These can
give you absolute position control much like a stepper, but
with much higher speeds and great torque.

Servo Systems Co.
115 Main Rd. PO Box 97
Montville, NJ 07045-0097
1-800-922-1103
(973)335-1007
fax (973)335-1661
web site http://www.servosystems.com

Typical deals:
Barber Coleman FYQF63310-9 geared 12 vdc 100 ma 500 rpm no load
torque at 160 rpm is 20 oz in at 850 ma. Cost is $9.50
I run these PWM and find them excellent for robotics.

Barber Coleman CYQM23061-5-2 geared motor with generator.
12 vdc 350 rpm 7.5 oz in 650 ma. Generator AC 3.3 v/1000 rpm
Cost is $17.50

Want something even cheaper? get your hands on the disgusting old
IBM 5 1/4" drives (the older the better!) The main drive motor
is 12 vdc with a built in AC generator. I use these for many
of our robotics projects. I usually get them for free, as
nobody wants them anymore. The more modern units use a brushless
dc pancake motor and so are not useable here, except for the
stepper motors.

Some surplus places will sell you an old IBM drive for about
$3 plus shipping.... but check for local suppliers first
and save the shipping cost.

{Quote hidden}

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