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'Futaba servo pulse widths'
1997\03\14@111944 by Brian Scearce

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Andy Kunz answers me:
> I make a living driving these servos, believe it or not.
>
> Center on "new" Futaba servos (connector on servo is a female) is 1500uS,
> with other limits at 1000uS and 2000uS.  Don't know where he came up with
> 1520.

My servos are the "new" ones.

I must be misunderstanding how these servos are supposed to work.

Unless I've misinterpreted you, you're saying that the 1ms-2ms pulse
width range is where the servo has linear response.  I thought that it
was the whole range of the shaft (i.e. just over 180 degrees).  For my
application[*], I don't care too much about linear response, but I do
care about having a large range of motion.  Can I feed the servo my
over- and under-sized pulses, or do I have to add some gearing to avoid
harming the servo?

I also figured the "center" was the middle of the shaft's output range,
but you're saying that the center is wherever the shaft is when you're
feeding the servo 1500uS pulses (which, since the output is linear in
the 1000uS-2000uS range, is the midpoint of the linear range).

Thanks for the help,
Brian

[*] For what it's worth, my application is a camera platform with two
servos mounted at 90 degrees to each other (left-right and up-down).  I
can put the camera against a wall so that there's nothing behind it,
but I still want it to be able to look all the way to the left and
right, so I need a full 180 degree range of motion.

1997\03\14@125533 by kotay

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>
> Andy Kunz answers me:
> > I make a living driving these servos, believe it or not.
> >
> > Center on "new" Futaba servos (connector on servo is a female) is 1500uS,
> > with other limits at 1000uS and 2000uS.  Don't know where he came up with
> > 1520.
>
> My servos are the "new" ones.
>
> I must be misunderstanding how these servos are supposed to work.
>
> Unless I've misinterpreted you, you're saying that the 1ms-2ms pulse
> width range is where the servo has linear response.  I thought that it
> was the whole range of the shaft (i.e. just over 180 degrees).  For my
> application[*], I don't care too much about linear response, but I do
> care about having a large range of motion.  Can I feed the servo my
> over- and under-sized pulses, or do I have to add some gearing to avoid
> harming the servo?
>
I would also like to know the answer to this question.  I have
always been told that most servos have 90 degrees of rotation
(except for the ones mentioned below), and that I should stay
within the 1-2ms spec.  I assumed that giving them out of spec
signals was not a good idea.

{Quote hidden}

I do robotics research and I need greater than 90 degrees of
rotation on my servos.  I found one commercial servo that has
170 degrees of rotation: the Airtronics 94739 proportional
retract servo.  There are also sail winch servos that have
rotations of multiple revolutions, but their positional
accuracy is poor (they add internal gearing to get more rotation).
I have been told that many connercial servos can be modified
to achieve a greater than 90 degree rotation.  The people at
FMA Direct will modify their servos to do 160 or 180 degrees
(depends on the servo) for an extra $10--a great deal since
their servos are less expensive than most anyway (the nature
of the modification is "a change to the amp").  I haven't
purchased any yet, but I probably will in the near future.
You can contact FMA Direct at:

FMA Direct
9607 Dr Perry Rd - Unit 109
Ijamsville, MD 21754
Email spam_OUTfmadirTakeThisOuTspamfmadirect.com
Sales (800) 343-2934
Technical service (301) 831-8980
Fax (301) 831-8987
http://www.fmadirect.com/


Keith

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

1997\03\14@145932 by Andy Kunz

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At 07:30 AM 3/14/97 -0800, you wrote:
>I must be misunderstanding how these servos are supposed to work.

They are only intended for 90 degree rotation.  The fact that they produce
270 degrees is due to the travel limits of the internal potentiometer.  If
you stay within 180 degrees, and away from a limit, you'll probably do fine.

Keep in mind, however, that not all brands, nor all models of Futaba, nor
even individual units, may provide the same performance when you are
running out-of-spec.  If you're only doing a one-off, you'll be just fine.

>I also figured the "center" was the middle of the shaft's output range,
>but you're saying that the center is wherever the shaft is when you're
>feeding the servo 1500uS pulses (which, since the output is linear in
>the 1000uS-2000uS range, is the midpoint of the linear range).

That's correct.  It shold be pretty close - how far off is it?

>[*] For what it's worth, my application is a camera platform with two
>servos mounted at 90 degrees to each other (left-right and up-down).  I
>can put the camera against a wall so that there's nothing behind it,
>but I still want it to be able to look all the way to the left and
>right, so I need a full 180 degree range of motion.

Interesting.  I had to do a pan and tilt for a video camera once, but it
had to be able to go round-and-round at whatever speed the operator wanted.
Now THAT was fun!

Andy

==================================================================
Andy Kunz - Montana Design - 409 S 6th St - Phillipsburg, NJ 08865
         Hardware & Software for Industry & R/C Hobbies
       "Go fast, turn right, and keep the wet side down!"
==================================================================

1997\03\14@171723 by taking

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Actually I was going to offer that it is very easy to modify a normal
servo to be a "spin all the way around" servo with little effort.
Remove the feedback to the pot, break the mechanical stop on the gear
(If there is one), and set the pot to midway.  A 1.5 ms pulse is stay
still, shorter is turn one way, longer turns the other way.  You lose
the positional feedback, but get a $5 mouse and use the opto encoder and
hook it back to the Pic (Ha!  back to topic!).  Cut one slot and you
even have an index for the opto so you have absolute positioning..



Andy Kunz wrote:
> Interesting.  I had to do a pan and tilt for a video camera once, but it
> had to be able to go round-and-round at whatever speed the operator wanted.
>  Now THAT was fun!
>
> Andy

1997\03\15@141852 by Andy Kunz

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>> <snip> Can I feed the servo my
>> over- and under-sized pulses, or do I have to add some gearing to avoid
>> harming the servo?
>>
>I would also like to know the answer to this question.  I have
>always been told that most servos have 90 degrees of rotation
>(except for the ones mentioned below), and that I should stay
>within the 1-2ms spec.  I assumed that giving them out of spec
>signals was not a good idea.

While it works, it is not recommended.  Operation out of spec may give
different results from different brands, different models, and even
different units of the same model.  That's why we have specs <G>

Most of the model airplane type servos will accept pulse from down around
100uS to well over 3000uS without going beserk.  They can also be refreshed
far faster than the 20mS spec.  Here's a free one for you - if you refresh
them faster, the servo has more apparent "torque" than one refreshed at
20mS.  This is because the motor drive circuitry is active more, and the
output cannot be changed as far before a correcting signal restores it.
Again, this is out of spec!

{Quote hidden}

Have you tried any of their other products yet?  What do you think?

Andy

==================================================================
Andy Kunz - Montana Design - 409 S 6th St - Phillipsburg, NJ 08865
         Hardware & Software for Industry & R/C Hobbies
       "Go fast, turn right, and keep the wet side down!"
==================================================================

1997\03\17@050251 by efoc

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Brian Scearce wrote:
> (Big Snip)
> [*] For what it's worth, my application is a camera platform with two
> servos mounted at 90 degrees to each other (left-right and up-down).  I
> can put the camera against a wall so that there's nothing behind it,
> but I still want it to be able to look all the way to the left and
> right, so I need a full 180 degree range of motion.


This is exactly what I designed my system for.

       I have a Conectix Qcam mounted on a system using 2 R/C servos to "Pan
and Tilt" the camera.

       It uses an Old Sega joypad to control the servos with the "fire"
buttons used to Store and recall pre-set positions.

       I built my camera frame out of Lego. My Daughter was not best pleased
when I glued the bricks together :)).

       In order to obtain an acurate movment my servos are mounted at the axis
of the camera at 90Deg to each other.

       I'd send you a pic but I only have the one camera.... maybe some clever
trick with mirrors or somit will work.

--

Cheers Peter ..........

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